I took a quick look at the 4412 specifications, and it looks like you're in the ballpark for CFM rating (500 CFM). The nest step down is a #7448, a 350 CFM that we used to use extensively on the round track engines. NASCAR specified that carb for engines up to 375 cubic inches to act sort of like a restrictor plate. A 350 Chevy will run 6200 RPM absolute maximum on that carb no matter what the gear ratio is! The 7448 will supply plenty of air to the 305 engine for any reasonable RPM range if you simply can't get the 4412 to work.
One thing to look at is the tension on the power valve spring. If it's got a pretty heavy spring on it, the power valve will open too quickly under load and make the carb run too rich regardless of the main jet size. A leaky diaphragm on the power valve will also make the engine run rich. If you need a few different power valves to experiment with I've got a race tuning box full of them, and probably 100 jets or more of different sizes. Drop me a snail mail address in a PM and I'll send you some. Good luck!
Jerry
