Yes, you need either a Downey or AA conversion bellhousing and probably a propeller type rear crossmember when installing a sbc, stk 4speed. I run sbc's in my 40's and use AA stuff. When you install a 350 using a stk FJ40 trans snd t/case you move the driveline forward several inches to clear the valve covers and to allow for air flow around the engine. You'll have to alter the trans hump and shifter cane because the shift tower interferes with the heater. It's my understanding you can use the FJ60 shifter top plate and the tranny hump hole lines up better and you may not have to alter it. If you use the Marks adaptor i believe the driveline stays in its stk location. I'm not really familiar with the Marks stuff. Your conversion really depends on what you're going to do with the 40. You can use many different trans a tcase behind a sbc, but you may have to change things downstream. You might want to look into a 5 speed choices too. I have a nv4500 in one and really enjoy it. If I were starting over I would look at nv4500, split case conversion. You get a solid 5 speed and a stronger tcase. The tcase has a lot aftermarket gearing options too.