I bought a brand new 75' EFI relay when they were still available and I still have it around haven't ever used it, I think I have an extra one from a 76 too but I'm not sure if that one works, If you do some research and the relays I have will work for you send me a pm and we can work something out. Also another thing to check if you haven't already is check the Injector resistors, they are on the driver side under the inspection lid, It shows you how to check them in the FSM I believe.
Good luck.