it looks like we are going to have to wait to upgrade to a newer MH so I would like to upgrade the Turbo Hydramatic 400 to the 700R trans. Has anyone done this here? Seems at 65 mph its topped out, so...
That would be a big job. Mounts would have to be created and something would have to activate the locking torque converter in the 4 speed that you don't have now. Have you considered getting an overdrive unit like Gear Vendors sells? They would probably have a kit to just bolt it in after shortening the drive shaft. My FIL had one and it worked automatically just like the forth gear in an auto.