Not a vacuum leak issue (you can tell by the idle RPM). Back firing is due to a lean spot in the accelerator pump curve or a timing issue. I see you have a vacuum acuated secondary with a vacuum can spring kit modification. If you have a stiffer spring then go up one or two colors to incease the vaccum differential before the back barrels start to open as they are coming in too soon.
Spring colors are by increasing stiffness.
You should have a silver spring in there now from the factory. If so try the brown one as that will shift the opening RPM from 1,960 RPM up to 2,380 RPM.
If you do not have a spring kit you can buy a HOLLEY SECONDARY DIAPHRAGM SPRING KIT P/N 20-13 from Summit, Jeggs, Speedway, Honest Charlies, or any other mail order speed shop.