if the 1958 is correct, that motor was never designed to run on ethanol gas, but probably won't hurt it but I don't really know. I would use the Stabil Marine version in every bit of gas you run thru that motor. Stabil only stabilizes the gas, and does not hurt the motor or any parts. Seafoam is more of a cleaning product IMO, ie carbon build up, etc. running some seafoam in the gas will help clean the carb, jets, things like that but I don't think it will it protect the motor from the ethanol in the gas. The stabil will help with that, by reducing the amount of water that is converted from the ethanol contained in the gas. I have some seafoam, and the can says fuel stabilizer, so it must do something to the ethanol fuel but i don't know what. If the motor is running rough, I would run some seafoam thru it. if you are wanting to stabilize the gas due to the ethanol content, I would use the Stabil marine version. Supposedly the startron stuff is good but have not used it.