The 78 would have had a 5.7. They were basically converted Olds engines. I'm not familiar enough with car kits to tell you what to look for, but if you could find a 350 Olds the conversion should be simple, especially in scale.
All old Chevrolet diesels that I've seen have factory duel exhaust that exits behind the tires.
You should be able to image search 5.7 Chevrolet Diesel, or 5.7 Olds diesel.