Help, engine misses under load

[MJN]

'72 Blazer with 350, HEI, misses under load and heavy acceleration, like going up a hill the engine will start to miss on one cylinder with a lot of hesitation and gets better when you let off gas. Seems to happen more around 2200 to 2800 RPM. I replaced cap, rotor, wires all new. Switched ingnition module with a good one, and adjusted carb and timing. Timing is set at 12 degrees before and vaccum at idle is around 16psi with idle set at 800 RPM in park. Last thing I can think of is that maybe one plug is bad or is going bad. Just replaced those in early August with AC R44T. Any ideas of what might be happening would be appreciated. Thanks!