ORIGINAL: riotgear " . . about the firewall/airframe. do you think a G-38 is too extreme?"
Some pilots have installed G-38's. Some others have complained about the sturdiness of the firewall. I had a mishap with my P-47 and the firewall collapsed on me - after I had epoxied all joints inside and out. As with the entire fuselage, the structure of the engine box is too weak for such a large heavy plane - engine. (Interestingly, I've found that the wings are very strong.)
To answer your question, I would say the G-38 is not too extreme based on what those using it have experenced. I put a G-26 in mine, and I really regret it. It flies okay, and will even do nice loops. But it has no reserve power to compensate for my dumb thumbs.
I would urge anybody building this kit - regardless of engine - to do some serious firewal/engine box strengthening.
Mike