| TH350's came in two lengths, 6" tailshaft and 9" tailshaft. The longer ones often came in trucks & vans.
If you use a 9" TH350, the driveshaft is the correct length. If you use a 6" TH350, you need a longer driveshaft, either one custom made, or one from an '82-'83 Camaro/Firebird with 3 spd auto or 4 spd manual tranny.
You also need some means of mounting the torque arm. A retrofit bracket is available that uses tailshaft housing bolts. Or, Spohn (sponsor button above) makes a nifty conversion crossmember and torque arm that takes the torque arm mount off the transmission (a very good idea, by the way).
The other problems you'll have is basically anything that the engine bolts to or anything that bolts to the engine is different between a V6 & V8. That's why donor cars are so highly recommended for this operation, even if you don't use the engine or transmission from the donor car. | 
