[Caddylackn]

What size Impco? If its a 425, that is only rated up to about 300 hp. You say this is a big block? It probably is not big enough to support your horsepower so I would imagine its running out of fuel at higher rpms. The Impco 425 is equivalent to a 2-barrel carb for a stock V8.

https://propanedepot.ca/wp-content/u...Carburetor.pdf

Watch your temperature gauge, as the motor will get hot and burn up valves if you are running it lean for long periods under load.