Hi Tim,
For my car I built a 289 with modern internals but trying to stay period correct. With aluminum heads, roller cam and stock style valve covers. I had to buy a spacer for the roller rockers, this fit on top of the heads between the head and valve cover. I used the original crank. I had to use a 67 timing chain cover as the 64 -65 did not fit with the harmonic balancer I picked out. I also used the early style
oil dipstick, they changed in 67 I believe. I did deviate from the original look with some after market March pulleys as I could not find a set that would line up appropriately. I even have an early style alternator case with new internals. My goal was to make the engine look period correct. The last item I need to change is the braided fuel line from the pump to the carb, I plan on modifying a steel line with the correct adapters.
So, it can be done, just have to pay attention to the little things that most folks won't even hone in on when they look at the motor.