Above article is the one I found but didn't link because it wasn't about "Mini." Even now, a Class C, even the 31/32-footers, are referred to as Mini Motor Homes in some circles.
This Jimmy could be a 454 V8 but I'd expect a 350 small block. There was also a "400" way back but I don't know if it was ever in vans and the WIKI article doesn't mention it. Transmission, a three-speed without locking torque converter or overdrive. Turbo Hydramatic, a "350" or "400." Those are transmission series names, not engine sizes. Still, typically the 350 was used with smaller engines and the 400 tended to go behind big blocks.
Very rugged engines and transmissions. Easy parts availability for the whole chassis.