After playing around with a 76 460 (4350 carb) and various Q-jets on boats, I found that unless you are after acceleration, you can ty-wrap the secondaries shut and get lots better mileage. On a heavy vehicle, even a fairly normal take-off can pull in the secondaries slightly, which dont mix all that well. The 4350 autolite carb had primaries way too small to run very much, but the hollys are ~300-350 cfm on primaries, which on a 460 will surprise you. Why dont you try ty-wraping (or some other method to totally disable) the secondaries and drive around for a while----its free, and you can tell just how much the secondaries are costing you. It will definately be slower, but at least you can try it. If you like it, then you might put a really stiff spring in the vacuum pull-up on the carb and keep the 4 bbl.