The 750 will work fine but if the car is mostly a street car the 650 would be a better choice. I have a 360 with pretty much the same build (a little less cam) and have had 600, 670, 725 and 750 carbs on it. In order of preference its the 670, 600, 750 and 725.
Getting back to your rich mixture at idle. What is your initial timing? You should have between 15 and 20 degrees BTDC with the cam you are using. The magnum heads like less (32-34) total timing than open chamber heads.
With the overlap in the cam you are using the cylinders are not filling very efficiency at idle and you need to start the fire earlier to ensure all the mixture is burnt. Advance the initial timing in small increments until you get max manifold vacuum. You will then need to limit the timing in the distributor to keep the 32-34 number. If you have a MP distributor or an aftermarket one then you it will be easy.
Remember the first step in dialing in a carb is to get the timing right.
