Yep, the C class with a 440 is the easy donor option. I've got a couple 440's from the bigger motorhomes, and the trannies have good parts inside, just the wrong tail shafts. Got them long before these engines became stupid expensive, $600 with trans was the most I paid, I'd never give that $2000 for the runner, or $1000 for a torn down block. But that's me living in the past when things were reasonably priced. Ah, the good old days!
@standup303 whatever route you choose, wish you the best of luck.