I reread this thread and your original post over in the 4-speed forum. You did some mods while you had the engine out but nothing associated with the lower rotating assembly. I rebuilt a 318 for my Barracuda, I had the shop balance it. They had me bring every lower rotating part to the shop to balance the engine, including the pressure plate and flywheel. You went to a 4-speed from an auto so your flywheel and pressure plate were not balanced with the crank, rods and balancer. I know the shop drilled a lot of little divots in my pressure plate to get it to balance to the engine. I’d be suspect of the rotating assembly not being perfectly balanced. It’s really the only thing that’s changed.