Claus, what is the alcohol type and content of your pump gas? Does your fuel contain oxygenates, and if so, what kind and how much?

Fuel quality makes a big difference in performance, and the gas sold here in the U.S. is pretty much garbage. New cars will deal with it; old cars don't.

I'd go with the 428, fill the first tank with 110 octane (leaded if no hardened seats) and drive it and tune it. When the tank is empty, put in half a tank of pump gas. See if there is a difference. If you've lost your power or have detonation, top the tank with 110. Adjust from there. It all depends on the accuracy of the builder's claims as to compression ratio. If the 455 is 11.3 there is no way it will run on pump gas.

Either engine can twist your convertible like a pretzel if you get on it too hard. That's why 428s were not installed in first gens.