I just got a 402 big block for my 81 el camino it has a turbo 400 transmission. Right now my el camino has a 350 and a turbo 350 transmission, i was wondering if the turbo 400 would hook right up to the driveshaft and work with out modifying anything.

The TH350 with a 12" tail is the same length as the TH400 with a 9" tail. Even then, the crossmember mount is a different distance from the back of the block.

http://www.transmissioncenter.net/TransmissiomMeasurements.htm

It would in most cases be worth the trouble of mods to make the th400 work. That is what the factory used behind the big block. Granted a properly prepaired th350 can be a tough trans. The fact that you already have the 400 would be enough for me to make it happen.