A 750 Thermoquad would work well on it, but I don't like having to stack up adapter plates. Since the Holley bolts right up and works and you like it, why not stick with it? Hard to beat the little 1850. If I was going to do something different, I'd look at the Quick Fuel Brawler in a 650 double pumper. They are some of the most tuneable carburetors on the market. But I think I'd really leave it alone. Have you worked on getting a good hot curve in the distributor?