Yes you can put a 6cyl trans behind a V8. Years ago when I worked in a trans shop we would keep rebuilt transmission on the shelf for change overs, all we would do is swap over the bell housings to suit. If driven like a everyday road car they would last a long long time (past the warranty period anyway).
Like Dattoman said the only difference is the number of friction plates between the 6 & 8 cyls which from memory is 2 in each clutch pack, also the spacer thickness to allow the extra plates.
All other internals are the same such as input / output shafts, gearsets and band.
Torque converter size is different also.