slant 6 / 318 swap

When you drop in the 318, the tranny mount in the car now should be use with ease to mount up the 904.
The drive shaft will need to be shortened about 2 inchs
The rear end, IMO, should be upgraded to a 8-1/4 over a 7-1/4. Though, the little rear will live with a stock 318, there known to give out with minimal power increases from a 318.
Exhaust manifolds can be a trick sometimes. It depends on where they exit.