Hi there, English:
With the 12-300-4, you are pulling a ton of vacuum, probably 18" to 20" of vacuum, which means when you open the throttle plate, air is going to rush in there like the thing was supercharged.
So you need a big shot from the accelerator pump early. Much richer than the standard curve on a stock carb.
Also, that cam has a ton of torque at low rpm. Torque, like horsepower, comes from how much air you are moving through the engine at a given rpm. So you are moving a lot of air at low rpm compared to most engines, so the shot has to be long, until the carb can catch up with the air flow.
I would start with at least two steps up from the stock cam, and set it on the richer of the three positions, and see what you get.