would it evan cycle with the gas port plugged
If the gas port is plugged it will not function the action.
However, I have shot thousands of cast bullets the last 40 years through numerous M1As, M1s, M1 carbines, ARs, FN/FALs, FN49s, SKSs, AKs and several others. I had one problem with one M1 Carbine because the gas port was chipped. None of the others gave any problems.
For cast bullet use in the M1/M1A I suggest the use of a heavier bullet than the Lee 312-155. For reliable functioning in many rifles the velocity is too high for best accuracy, especially with the M1.
A heavier bullet allows medium and slow burning powders to be used while keeping velocities at an accuracy level of 1850-2200 fps. How fast depends on the twist, A 10" twist will get best accuracy in the 1800-1950 fps range and 12" twist will get the same accuracy up to 2100-2200 fps. The heavier bullet with medium/slower burning powders make for workable gasport pressures that will function the actions. The best and easiest to use in the M1/M1A for accuracy and functioning cast bullet I've found is the Lyman 311299 or the 314299. The powder I use is 4895 in both the 7.62 and the 30-06. I also use a dacron filler for better consistent ignition and accuracy.
Larry Gibson