I've got my 73' ford with a 390 running on duel fuel, propane and gas. It runs great on gas, but when you switch it to propane, it stumbles. It will only do it right off idle while under a load. It acts like it is running out of fuel, or not gettng enough fuel. It runs great at every other rpm. I just rebuilt the mixer and its got a good regulator on it. I've tried adjusting timing, and the power valve, and i've messed with the idle mixture, but nothing helps, maybe someone on here has an idea what is going on with it. Its got an Impco model E rugulator and Impco mixer.
Is it a Imperial 300? Or is a imperial 300 50/70 (it will be marked in the top cover) If its a plain ole 300 it isnt really big enough for that 390. But either way ther is a load screw in the side of the elbow try it full rich and see if that helps. Also make sure you do not have any air leaks like a missing gasket between the carb and adaptor or a cracked hose