I have a slightly modified 302, 351 D0 small chamber heads, oversized valves w/hardened seats, slightly agreesive cam, Edelbrock Performer manifold and 600 cfm carb. I'm running rich post modifications. I am slightly concerned I sucked up some rust from the tank and it's leaving something open in the carb, but I'm also wondering if I need a hotter spark plug. I'm running with Autolite 45s. Pre head and cam swap was running fine.

 

Pull the air horn off and check for any debris that may be hanging up the needle/seat, check/adjust the floats while you're in there.

 

Are you suggesting that the hotter plug will change the air fuel ratio? Hotter plugs do not create a hotter spark, the ignition output cant vary based upon the plug. The plugs heat range is a temerature range in which heat is transfered into the head. This is a reference to how much of the insulator is exposed.
This exposed insulator determines how much heat gets tranfered into the head and is permitted to be exchanged with the cooling system, and not a reference to the ingition voltage.
You can only create a "hotter spark" by modifying the ignition system and oncreasing voltage output.

Ok, so to the problem. It is likely that you simply have a rich condition partly due to the larger cam, increased duration, lack of vacuum, and the basic principles of how the carb works.

Most of the time, the rich condition occurs due to larger camshafts and tha mount of throttle position is required to keep the engine at idle. See since there is less vacuum, due to the added overlap and duration of the more aggressive cam, the bosster signal is weak. To compensate for this lack of air, idle speed has to be increased, and by doing so, one simply tweeks on the idle screw to open the blades right? Well, this is not exactly correct. By doing so, the blades can be opend past the transfer slot, and exposes too much of this transfer, and the carb no longer idles on the correct circuit.
Once the blades are opened too far, the carb thinks that the trottle is open, which it is, and the primary circuit is now being used.
Ok, so we are trying to idle on the primary circuit and this is calibrated for much more air than you have at idle, and the condition that you will get is rich, real rich.
This is where high performance carbs start to shine, as these are typically adjustable in this area.

This is just a hunch, and a fairl educated guess regarding your carb, however, there is a possibility that you have damaged the power valve during the engines initial start. It is also possible that you have dirty idle air bleeds, or even some idle emmulsions that are clogged and you are not permitting any air to mix with the fuel.
Any of which are possible, but I really suspect that the throttle blades are open too far, and that you are trying to idle on the primary circuit.