Where are the spark plugs in a '98 LS 400? Are they under those black sqare boxes that are held down by a 10mm hex? Do you just pull and pull and pull really hard until they come Off? gone are the days of my '72 VW bug and my '69 Mach one...

 

G - Yes, the plugs R behind those boxes ( which are the individual coil packs for each cylinder ). Just disconnect the Molex plug connecting the coil to the spark plug wire then U'll need to gently but firmly pull upwards on the boot. It'll come free ( may have to twist it slightly ) then U can reach the plugs with a long ( 10" ) extension & a 16mm ( I think? ) deep socket. Just be careful reinstalling the new plugs because the head's aluminium threads can be cross-threaded easily.
Mach One, huh!? Didja have a Boss 302 or 351 Cleveland? Those were the days - had a '69 Z-28 ( with a dual Quadrajetted ) 302 myself.

 

351 cleveland. where I grew up, one the "funnest" things I did was, Friday afternoon, after school, I wuld drop my headers and run up and down the backroads. Spitting gravel, those L60's cut a pretty wide path!!! I'd come back home about 10:00 pm, turn on the porch light and reconnect those headers, go in the kitchen and make a big 'ol pot of home-made french fries!! I'm starting to tear up....gotta go....

 

Yes, the plugs are under those two plastic cover with a bunch of 10 mm bolts. The driver side is easy to take off. The pass. side is a little trickier to take out. You have to take the intake hose off to get to it. be careful not to brake any of the plastic hose connector and the plastic cover on the pass. side

Good luck