Jeff, I just read your other post about the difference between a 350 and a 400 distributor. I did not read it earlier as I don't know what the differences are other than the timing curve as others have stated. Seems you have two questions [Posts] regarding the same problem.
So far, if I read things correctly, you have a problem with backfiring through the carb, a classic timing chain scenario. I once spent from Christmas eve to Jan 2nd on the side of the road north of Campbell River waiting for the parts stores to open so I could buy a timing set for a Chev truck. Same problem, backfiring out the carb. Luckily we had a camper on the box, some fly rods, Salmon to catch and plenty of Christmas cheer to pass the time. So far you have: Changed the distributor to a used one off a 400; Put in new plugs, and wires; Installed a new timing set; Installed a vacuum gauge and have a jumping needle situation. You still have the same backfire problems you had before any of the above was done; You are considering a Moroso part to possibly alleviate your problem.
Please do not buy any more parts until you have diagnosed your condition and know, within a certain amount of confidence, what the actual cause of the condition is.
If your vacuum gauge is showing a needle that drops and comes back to the normal reading every couple of revolutions, that could be a sticky valve situation, The needle jumping around and not settling could be an indication of a blown head gasket, as well as camshaft lobe wear.
Have you done a cylinder compression test? Do you have any oil in your coolant, do you have any coolant in your oil?
Setting the idle mixture is different depending on the carb and what you have for accessories. Some start with two turns out, some with five turns. You are able to start the engine and let it idle so, set the idle speed to 700 RPM, turn the left mixture screw in or out until the highest vacuum in obtained. Reset the idle. Adjust the right mixture screw as you did the left. Continue adjusting the two mixture screws and the idle screw until you obtain the highest vacuum possible.
