A old trick that still applies was to set the lifters the minimal amount to keep them from ticking. The old engines would "float" at high rpm when set to factory lifter spec's. There wasn't enough clearance/oil in the lifter when set deep to keep the lifter from floating off the cam and holding the valves open at high rpm. So by backing the adjustment off, you could gain clearance and more oil for slightly high rpm before it would start "floating" again.
3/4 turn after zero lash is a good starting point. If the lifters are quite at that setting, going deeper is not going to change anything. To deep and you can hold a valve open causing some running issues, to little and you get the ticks which shouldn't hurt fuel milage, but still should be taken care of.
Check the carb gaskets as suggested, and vacuum leaks. With all the work done to the engine, fuel milage sounds like it wasn't an issue. Dual carbs are fuel eaters if not sized right for the engine. No amount of tuning will cure that. Cam choice is another factor. Head work doesn't seem to effect fuel milage unless it was opened up way more then needed.
Keep us posted on you out come, Joe
