Timing cover and intake are the ones I comonly see leak, I wouldn't bother with the others they are pretty bullet proof. I see more replacement oil pumps go bad than the old factory ones, don't bother with it either. Throw in a double roller chain set and advance the cam 3*, that heavy truck will like a few extra pounds of TQ. Autozone and Oreilley can get them cheap with the adjustable slots, tell them it is a 68 340 dart or they will never find it. If you pull the heads you will see cracks and not feel comfortable running it so just leave em alone, if you do pull them put on the thin mr gasket head gaskets for a small compression bump.