Have you seen this transplant on the same generation Truck as the '89?
I don't know, if it were me, and if the cab is in good shape, I'd retain the cab and then do the engine and 4wd swap if needed. I'm a sucker for those classic lines and the quad round headlights.
It would be a bit of work to prep a pickup chassis for the Chinook body. I would hazard a guess that an engine swap and 4wd conversion might be easier.
The 3VZ-E 3.0L in the '89 isn't considered Toyota's best engine, but it's really not that bad. It carries the much larger Toyota RVs around just fine and is long-lasting. It sounds like the head gasket issues have pretty well disappeared by now.
Still, consider keeping that '77 cab-chassis under it. That vintage look is tough to beat:
Have a look at Nolan's build (the brown one). There's a lot you can do with the 20R and the original cab-chassis:
http://www.toyotamotorhomes.com/nolan's_76_chinook.htm