I have seen a strait 6 ford that looked just like a gas one,but it had the injectors where the plugs would be,it was on a big generator. About the olds,I think it was likely a 350,they look like a 455. They were sort of neat in the respect that the later year ones had a stronger block, 3 compression ring forged flat tops,real beefy rods,455 size mains and a roller cam,they could be changed to gas by putting the gas heads and a intake on them and regrinding the cam. The gas heads don't flow that well though, but they made for a bulletproof truck motor,great for a diesel retrofit.
I have a friend who is thinking about keeping diesel heads on one,but putting a gas intake with a distributer on,setting the heads up for plugs and running alcohol at 20 to 1 compression,not as a go fast,but for economy.