The 75 engine should work in the 78, just use the 78 harness and any attachments and accessories off the 78 engine. In other words use basically bare 75 engine block, head and injectors, and fuel lines and connect them to the 78 f/i, and ignition harness. Use the 78 alternator (it is internally regulated). There may be some emissions related temp controls etc on the 75 that are not on the 78. You should be able to eliminate most of them. Just take any of the odds and ends from the 78 engine and put them on the 75 engine. Then you can continue to use the 78 ecu.