Gotta wonder about the 904. Is it there because it was available, or is there because someone wanted a lighter transmission and did the work needed for it to survive racing and hard use?
Lots of good suggestions in this thread.
I'm with A-bodyJoe - install the diff already owned, and run it.
And with A/J et al to then adjust the timing and/or get better fuel.
Drive it and see how it is.
Honestly, if there was no time crunch, and there obviously isn't, I'd start with a simple compression check and then measure cam lift with a dial indicator and preferably a degree wheel.
