A donor vehicle of the right vintage would definitely make things easier. When I picked up my 4L80E (from a friend) I managed to at least get the computer that goes with it so aside from a wiring harness and adding some sensors I should be ok.
If you're going to spend the money to convert, diesel isn't a bad way to go. If I hadn't had the donor 454/4L80E combo I might have considered diesel but that's water under the bridge at this point.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)