you gotta poke around, he probably did.
Does the current carb have vac secondaries? If so I'd keep the holley and add a kit that allows you to change the secondary opening spring. Add a heavier spring if you want to save some gas.
Kit looks like this, the black thing in the foreground. 4 screws, change spring, done.
hey, that 600 right there might be for sale in a few months, lol