Sounds similar to a problem I had with my 58, 235 with a Rochester B. In my case, I could start the engine with no problems, but once I pushed the choke all the way in it would misfire with an occasional backfire, under load. It would run fine if I left the choke partially out. I played and tinkered with the carb to no avail. Then I checked the timing. It was the timing. I still need to replace the vacuum advance, but the timing is currently set so that I have no problems during my normal driving. I suppose if I were operating under a real heavy load, it would still misfire given the vacuum advance is inoperative.
