Mick, what Impco are you running? I ran a Impco 425 on a Holley 650 - 750 (same plate) baseplate, on a 351c in a 82 TD, and also on the 408 in my F150, with no issues.
When i fitted the supercharger to my F150, I ran 2 x 425's on an adaptor, to a single baseplate. I did buy an 850cfm baseplate, but never fitted it, before getting a bigger blower.
If you are running dual fuel, I'd run a 425 on a snorkle, not a mixing plate.