Mutt,
I had a reply typed earlier, but didn't want to be negative if someone else had made this work. All of the OEM evaporator/heater boxes are made to fit the space, to fit the evap & heater core, but more to distribute the air without need for many hoses to minimize labor on the assembly line. Every make & model, year to year is different, and like Howard says they are huge boxes. A person that knows auto HVAC might be able to gut an OEM box, fabricate a new box to fit the space, and adapt connectors for duct hoses to route the air but man, it would be a chore and might not ever work right. Vintage Air will sell you pieces/parts as you need them, and if your compressor's good no reason not to use it. Same with the condenser core, if it fits the space for you.