Never saw one in a g-body, but basically the fan just needs to properly pull air through the heater coil to function. In an old panel van work truck, the core mounted in a sheetmetal mount drew in air from the cab &the fan in a box under the hood area blew it back to the vents. If you can easily fab sheetmetal you could make something . The heater core could probably pretty much just sit where it currently is, remove portion of plastic box where ac coil is & fill back in. Heat & ac coils are inline with each other, each being shut off & on as needed. You WILL still have some stuff under the hood unless you find an old aftermarket unit to fabricate into the inside space under the dash, like an aftermarker ac/heat unit would.
Junkyard panel trucks may be a source for cheap.
Hope this gives you a little direction.