The later th 350's were better because they used more Torrington type thrust bearings as opposed to the conventional thrust washers. It just puts fewer wear particles in the oil which causes less problems for the valves in the transmission. I think the change was made gradually between '75 and '77. There were some small improvements made in the early years of the th350. There are heavy duty and light duty versions of the transmission. The heavy duty had a few extra clutches here and there. The light duty transmissions can be easily modified to do the same.

The final, most important difference to look for depends on whether or not you are running floor shift or column shift. If you run floor shift, you have to get a transmission with the mounting bosses for the floor shift mechanism on the drivers side of the transmission right above the pan.