Hi Matt,
I'm working on a DOHC/MT 75 build as well, just haven't got as far as you yet.
If you're having the propshaft made up by a specialist, I'd contact them and ask about it - no harm in letting the professionals explain it! Alternatively, I think "axle" on this forum has already completed his DOHC car - maybe he can explain how he did it?
Regards, Mike.
For a bit more help/info though:
1) This question has been raised on Locostbuilders a few times, with no real conclusions! A lot of the threads refer to
this webpage.
Scroll to the bottom of the page, and you'll see what limits the page's author puts on the propshaft angles. However the working angle limit of 4 degrees only covers a max. of 3500rpm; the DOHC engine at full whack in 5th gear can be doing almost twice that!
3) Don't forget to consider lateral offset of the propshaft as well. If you're looking directly down on the car, you want the engine and diff to be (almost) perfectly in line (if that's possible). Otherwise, the propshaft angle will be greater than you've calculated.