Forgive my ignorance please and don't beat me up because I have a STOCK 1954 4 barrel T-Pot on top of a stock 256 coupled to a stock 3 speed with overdrive.

I have read countless time on forums that the power valve in the carb can be damaged from backfires thru the carb.  During initial timing and start-up I experienced many backfires, eventually getting it to run reasonably well.  The carb has been rebuilt and the idle mixture screws have been adjusted by using a vacuum gauge.  The float level is properly set and the auto choke appears to be working properly.  The main jets are appropriate for the local elevation.

Now, the problem.  Under load (pulling my 2,000 lb. vintage trailer up a grade) the engine coughs with too much throttle in third, I back off and shift into second with less throttle and it smooths out quite a lot but not completely.  The coughing begins immediately if I give it more throttle.  Could this issue be related to a failure of the power valve?

Now the ignorance!  I have not opened the carb up to inspect the power valve but, when I do, is it glaringly obvious that a backfire ruined it or is the damage more subtle?  How can I tell if it needs to be replaced?