Thanks for the replies regarding the GP Cherokee.

I was able to see the same plane currently being built that is powered by the OS-52 4 stroke. As I suspected, the engine is a very tight fit, as the rear of the engine is against the firewall and the drive washer barely exits the cowl. Therefore, I will be moving the firewall back 1/2" and that will provide the room needed to install the OS-52 with a little room to spare.

DaveB