A truck is typically too heavy for a double pumper because it's likely to bog on acceleration. The 600 will have better air velocity, which will make carburetor jetting and adjustments more effective. You could possibly reduce the orifice size on the discharge nozzles to make it work. I'd get in touch with one of the Holley Techs and go through some rounds of tuning on the 650. You've made some upgrades, so the 650 could have sufficient air velocity also. I run a 670 Avenger, and I had to reduce the pump discharge nozzle to eliminate an acceleration bog. It was also because I'm at 5,000 feet elevation or higher.
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