Where you put the prop flange on the RV-4 is up to you- 39 inches from
the firewall is stock length, IIRC, but think of where you might want it
if lightweight, ie, aluminum end and middle housing engines become
available.

One recommendation I'd make, and it was made to me by Everett Hatch, is
to make the spinner diameter bigger. I chose 16 inches. That also
provides a lot more room around the sides of the engine. Room is what
you are short of with the RV-4 installation. Give yourself every
advantage you can.

Brian Trubee

Speaking of that I think putting the oil cooler on the
cool side of the engine leaves more room underneath
for a wider radiator. You will need louvers in the side
of the cowl.