Steve,
I kit bashed a Sig Magnum to a P-40 and it worked out great! (Except for the weight - that was back 16 years ago when I was still building with steel plate. Or so it seemed.)
The only real challenge was the canopy change and the cowling, but it was a fun and successful project. Powered it with my old Merco 61, a great choice.
Blessings,
Will