I want to clear up some confusion here. I did A LOT of research on bolting the newer LS motor to the older style TH400 transmission during my swap, most of which just confused me even more. The truth is, this is not only possible, but quite easy to do. Read more to see how.
If you get a 4.8/5.3 engine with a dished flexplate the ONLY part you need is a Hughes Performance HP3795 Transmission Adapter. The reason you need this is because the LS motors have a crank that is .4 inches shorter than the older Chevy engines. The dished style of the flex plate makes for the same bolt pattern, but the Hughes flexplate adapter spaces the crank out .4″ to make up for the shorter crank.
Warning, make sure you use Locktite on the 6 flexplate bolts and follow proper torque sequence alternating side to side.
First time through torque bolts to 15 ft/lbs
Second round torque to 37 ft/lbs
Third and final round torque to 74 ft/lbs
This is an excerpt from my Ol’ Blue LS Swap buildup because I still have a lot of people that ask me about it and it was buried inside my buildup. You can read about the rest of the LS Swap into my ’71 Chevy here: 5.3 LS Engine Swap into Ol’ Blue 1971 Chevy – Part 1
You can find the LS adapter on Amazon for pretty cheap considering how easy it is to install and that no other additional parts are needed.