I like the Edelbrock 1806 for this. That's the AVS Thunder in 650 CFM. The AVS is very similar to the Thermoquad, in that its rear air door is positioned over the boosters. This gives a much smoother transition into the secondaries and is much more tunable and much more forgiving. The carburetor has total vacuum secondary operation like a Thermoquad and the engine will only see as much CFM as it demands, up to its 650 CFM limit. No need to worry about it being too big, as it is very tunable on the primary side. I like the 650 here, because it gives you vast room for growth in the future. If you choose only the 500, then you are limiting yourself to a very mild, small cubic engine. Choosing the 650 will not hurt the little motor now and will give you a carburetor that you can use as your engine grows and you can even use it on a mild to moderate 340 or 360 should you choose to do a swap later on. Just my 2 cents.
