[71Chev]

71 was the first year for 5 lug and also disc brakes. Maybe it's a 71-72 rebuilt as a 70? Maybe redrilled for 5lug wheels, I don't know. A 454 was never installed in a 67-72 truck from the factory, not an option.

I think someone has pulled your leg. Over the last 30 yrs someone has swaped in the 454 and put a dual propane set up on it so it can switch from gas to propane, for fuel economy.