I commend you on this daunting task!
I am at 202k and I'm slowly getting mentally prepared to DIY this on my own...still deciding about either having my manifolds repair-welded or just get new ones and sell the old ones...which would mainly be driven by the fact that my starter may be original since I purchased the car over 12 years ago at 83k miles.
I guess the thing that concerns me the most is making sure I reconnect the subframe and making sure it's properly aligned.

 

Had the OEM manifold rewelded at 100k miles, it was only the passenger side that cracked, the driver side manifold was surprisingly intact, no issues. Then the rewelded passenger side cracked again after 10-20k miles, so i drove it for another 40-50k miles like that, no CELs at all, no noticeable power loss either. Replaced both sides with OEM + OEM gaskets, the exhaust actually sounds slightly louder and more of a deeper bass, just by replacing the manifolds. Power wise, feels the same, but “smoother” upon acceleration now, but could be psychological vs actual instrumented tests. I’ll be keeping the old “rewelded” OEM manifolds as a spare, for the next time this happens, and with the cost of the manifolds increasing every year.

It’s daunting for sure, but doable. Once the job gets started, it’ll be easy to figure out what needs to come off to get to the manifolds. There’s an old 40min Sikky header install youtube video that helps as a reference. Reinstalling the subframe, I marked the bolts and subframe to the chassis in various points, as much as I could with a grease pen, so the reinstall wasn’t so bad, alignment afterwards still recommended but not detrimental.

 

It's better to assume its not on straight and align it yourself. There is a center brace in the chassis with four bolts If you measure from the bottom front of the subframe underneath the center of the LCA bolt hole in an X to the front pair of those four bolts in the center of the car, you will get your subframe aligned pretty dead on. It will be both square with chassis center line and centered left to right.

When I did this job, I completely dropped the subframe to get it out of the way, so putting it back correctly was on top of my list of things to accomplish.

I had planned on making a video of this process, but its just about impossible to shoot a worthwhile video while on jackstands. I need to visit my dad and use his lift to make a video.

Here's a diagram that might be helpful. It shows the locations of the measurements both front and rear. Measuring in the X format makes it harder to get it wrong.

 

What did you use to support the engine when the subframe was unbolted?
What combination of extensions and sockets were used to take manifold bolts off?
Any other tips to share when doing this on jack stands?

I'm about to DIY this in a few weeks.

 

I used a hydraulic jack stand to support the engine at the transmission bell housing, before lowering the subframe, by removing the two huge long bolts and the 2 nuts.

I used a GearWrench 120xp 3/8 ratchet for pretty much all 16 manifold nuts, with a variety of 3/8 wobble extensions. This wrench really helps since swing arc is much finer than normal ratchets, esp when working in tight spaces trying to turn the wrench. Once the heat shields are removed, it’s pretty clear on which extensions are needed to reach the manifold nuts. Passenger side was much easier to reach all the manifold nuts. The driver side manifold had one nut that was semi blocked by the ac condenser lines. Had to use a very short and thin walled 3/8 socket with wobble extensions to reach this one. It will make more sense once start working on it and what needs to come off. Engine dipstick tube also needs to be removed, when reinstalling make sure it’s seated well and use a new o-ring for the bottom of the dipstick tube where it’s inserted into the block, and try to clean that area well from debris. Daunting but doable. Wish I could afford a lift, would have been so much easier.

I used this jack to support the trans/engine at the bell housing, but I suppose one could also use a hydraulic jack to do the same thing, just make sure safety first and have redundancy. Once the jack is in place though, supporting the weight of the engine, it will be in the way, in the middle of the work area. It’s doable, not enjoyable. I would have preferred to support the engine via topside, but the isf engine hangars are obscenely priced at $600 each or something now.

https://a.co/d/hINnstJ

 

I lowered the engine onto a jackstand with only the front of the car off the ground. You'll have a lot more room to work with just one end lifted if you are on jackstands.

 

If you removed the steering shaft, what did you use to lock the steering wheel straight?

 

The steering wheel locks itself when you shut the car off. The real trick is getting it reconnected properly once the rack is disconnected. Lexus tells you to make a match mark on the shaft to align with the splines on the rack. That's a lot easier said than done. When I put my headers on, my steering wasn't straight, and I ended up putting the car back in the air and turning it on, but not starting the engine, and turning full lock left to full lock right and looking at the position of the steering wheel in these two positions. When you get it right, you'll see the wheel clocked evenly at both positions. This is easy to do with the front of the car off the ground while the engine is cold (the headers are right there to burn you brutally if you try this with a hot car). It took a couple of iterations, but it's pretty obvious when you don't have an even split to left and right from center. FWIW, this was after a header install, so even less room to work with in the steering gear department.

 

Ditto what lobux stated above. Went through the exact same thing to get the spline recentered.

 

This is a great note here. You have no choice but to remove the steering shaft with the equal length headers. I am not looking forward to getting that reattached soon.

 

IIRC, you have to remove the steering shaft no matter what headers you install.

 

I didn't completely remove it. It pushes up pretty easily once you disconnect the rack. It's designed to do that so you don't get mashed with the steering wheel in a severe frontal collision.

 

I should have more clear on this. What I meant was to loosen and slide the the yoke sub-assembly out of the way. It's been several years since I did this...

 

What's the most appropriate method to do this? Measuring tape via center of each bolt?

I will be lowering the subframe with the engine (valve cover seals, long tube headers, starter, alternator).
I have access to the auto shop at the AFB near me and I've been doing my own alignments. I imagine I'd loosen all four plates and the appropriate subframe bolts, then shift them as needed (pry bar? mallet?) without knocking the car off the rack.

 

That's how I did it. Bolt center - ideally put the bolt in a lathe and make a divot in the center for measuring.