Wow, sorry to hear about your engine problems.
I would buy a crate motor form GM and have it installed. I believe they have a decent warranty and turn around time would probably be faster that way.
There will be nothing economical about the replacement. Its a lot of work to pull the engine and replace/rebuild it. I'm in the process of doing it on my 74 Argosy motorhome and its a chore to say the least.
If you wanted better performance and have the money then a 502 crate motor with fuel injection and a lot of cooling fans would probably a nice upgrade.
I would stay away from a diesel conversion due to the excessive cost and fair amount of changes that have to take place.
Air forums # 1674
1974 20' Argosy Motor Home
1975 24' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels & windows left to sell)