octane for this combo..

[Adger Smith]

When changing fuel you need to study the specific gravity of the 2 fuels. It might require a jet change just to get the same flow through your jets. The Air/fuel ratio might change. The specific gravity also is a factor in the lbs per hour of fuel the engine sees. Some fuels require more or less lbs per hour to make HP than others. This makes it important to do a jetting loop and timing loop to evaluate the fuel & octane needs of your engine. In short: just draining your fuel and adding another isn't a good test. You may be leaving power in the can by not evaluating what makes your engine happy with the fuels......
I might add that spark plug location and chamber shape has a large effect on the timing that is required. I'm not up on what changes Edelbrock did on those heads, but the best tueup may have no relation to stock chamber iron heads.