Big block / small block
You can find almost anything. Plus, with a name like "Farmall1938" I'd figure you'd be used to it ;). I was at the first fair of the season yesterday, watching a couple Farmalls, and a few Olivers pulling it out with the Internationals and Deeres.
Anyway, If you have a clutchflite, I dont know which would be easier, but i'd think a larger engine, smaller bell might be the best way to start... So I'd look for the 440s.