I believe that in 1968, when you bought a 350ci engine and wanted automatic, you got the th350, and the 400ci got the th400. But I think it could have been an option to get the 350ci w/ th400. But no option the other way around. And yes a 400ci will bolt right up to a th350. The only real differences in the th350 and th400 are the torque that they could handle(th350 550tq, and th400 850tq), and the tranny pan.