A-904 Interchange

Really enjoying my 273-powered 1967 Dart Convertible, but I've realized that I miss part-throttle downshift from my previous (newer) Mopars from the 70's.

I have access to a 904 from a '74 318 Charger, but I vaguely recall something about a flexplate or torque convertor change between '67 and later years.

What's the easiest way to get part-throttle downshift with the parts that I have? Will the later trans swap as they are, or can I rob the valve body and/or other parts from the '74 trans into my current trans?

-vm