You are in the exact same position i was in, I bought a 440 and a transmissionout of a motor home that had 41000 original miles on it, took it to the engine shop and they said it looked like it had just been rebuilt, I had to get a different transmission, even if you change the tail shaft the transmission still will not fit. my car was also a 318, I installed a set of bigger tortion bars off of a 71 r/t, .96 dia.....i think, im not sure about the driveshaft, find an interchange book and figgure out what other driveshaft might fit, you dont have to have one off of a big block automatic, i have a 440 automaitc in my car with a 8 3/4 rear end and the driveshaft off of a 74 318 3 speed manual transmission fit in my car.