Transmission and rear end combo

I have a friend with a 94 dakota and put it into a wall. He has a 93 318 in it with either a 500 or 518 and has a possi 8 1/4. I can get the trans and rear for $300 but am unsure if I want the trans.
Right now i have a 73 340 built (hyper pistons, bored .040, 214 @ 214 cam. 360 j heads, 2.02 intake, 1.68 ex, 700 cfm eddy, hydra lifters, and other stuff), my car has a 904 for the 340 and a one tire fire 8 1/4 rear.
I know i want the rear. I'll just swap the guts, but the transmission is what I'm worried about it.

Can some one tell me pro and cons about the 904 and 500 or 518?
It is a street some strip 65 barracuda.
Thanks
Theren