360 converter

66 Barracuda - nice!!
8 1/4 rear is pretty strong and will handle 360 torque, it's hard to get enough traction to break it. Does it have the same 4" bolt circle for wheels? 7 1/4 might hold up.
64 to 66 273 engines had a small diameter pilot hole in the crankshaft for the torque converter hub to fit into. The 360 crank has a larger pilot hole. All 360 ballanced torque converters have the larger hub diameter.
B&M makes a ballanced flex plate for a 360 with a 904 trans and will fix your vibration; Summit Auto Parts sells it, about $80.00. You than need to have a bushing made to fit in the 360 crank to center up the smaller hub of your original 904 converter. Worked for me.
Rick