I kind of figured cost recovery might not be high on the list. Still, there are some decent 6.2 blazers out there. 5 minutes on autotrader found a pretty good prospect by the description:
http://www.autotrader.com/fyc/vdp.js...=&cardist=1391
It's in florida, but i'm sure more are out there. Even so, going with a 4bt is certainly an option if you can fab it up. Here's a thought as far as motor mounts go. Try to get the mounts and the engine cross member from a 4bt equipped chevy p-chassis step van. Reason being is that the front half of the frame on the P-chassis is very similar (if not the same from what I've been told) to the 73-87 as they came out at the same time. You should be able to literally bolt in the mounts with little to no fabwork at all.
Frito Lay was one of the big users of the p-chassis with the 4bt so thats a good place to start looking. One thing though, those things don't die. Up until recently Frito actually rebuilt every step van in its national fleet on a regular schedule. (something like 5 years or so- can't remember). Since the aluminum bodies didn't rust they basically replaced everything in it but the frame. New suspension, engine, trans, brakes, axles and so on. Frito usually rebuilt them at a location in Dallas TX. They might be a good source for the used parts or at least where they send the old ones to the graveyard to obtain the x-member.
Frito is getting out of step vans and not rebuilding them in order to go to a more "throw-away" fleet. Meaning, they might be selling off the old stuff as it comes to the end of it's life cycle. They might only do this in Dallas or at main distribution centers in other cities. I don't know. But it might be worth checking into to make the conversion a little simpler.