Got to have that torque converter able to lock up in a 700R4. I have seen them burn trucks down when not operating right. The torque converters generate a lot more heat in a 700R4 than the 350, 400, even the 2004R. They are also longer than a short tail 350 so a drive shaft has to be made to fit. A 2004R built right will hold up well under a 350 and is so close to the same lenght as a TH350 it will bolt right in. Simple to wire up for the lockup torque converter, and more forgiving on the detent cable adjustment.
