Hi Mike,
Sorry for delayed reply, just been away from computer. I put a magnum 91 in a modeltech p51 for my cousin. (It was just a plane I put together for him). he still has it, it was a while ago when i did but it didnt require any major mods just was tight, I do remember having to put the battery in the rear of fueslage to balance but other than that wasnt a big deal, I can't remember how the firewall was on that plane but I know I didnt have to change anything major. Anyway sorry for the delay
scott