Here is one of those threads in which I posted information about towing capacity: http://www.clublexus.com/forums/ls40...8-trailer.html
With a transmission cooler, the documented towing capacity of the LS400 sold in Europe was 2,000 kg which is 4409.25 pounds.
I bought the tow hitch and wiring harness for my last (2000) LS400 in 2003 from etrailer.com in Wentzville, Missouri. The first thing I did after I bought a Sienna van in April to replace the 2000 LS400 was to buy a hitch and harness for it from etrailer.com.
Etrailer currently has a 3,500 pound capacity Curt hitch for your LS400. It's pretty easy to install - just drop the mufflers and bolt the hitch above the muffler brackets. The trailer wiring harness should not be powered directly from the car's tail lights - there is a hot terminal to the left side of the engine compartment fuse box perfect for powering trailer lights. I had a mechanic run the trailer harness power wire out an ABS wire hold in the trunk underneath the car to the engine compartment.
I think you will need a transmission cooler if you are going to tow your boat - it will likely be well over 2,000 pounds fully loaded including the trailer.