Sounds like the main problem is a major bog with backfiring after a few (or more) seconds at full throttle. That points to the fuel pump like I mentioned on the first page of the thread, especially with the description that was quoted by Jim (thanks, Jim!).

Having it happen at the top of each gear could be a fuel pump that is at its limits for your combo. I'd start the testing with a fuel pressure gauge hooked into your fuel line to the carb.

The stock intake (dual plane) has two plenums and each feeds the outside two plugs on one bank of the engine and the inside two on the other bank. If you can remember which ones were where like Vikki said or check the new ones for colour, you may be able to isolate which side of the carb may be dumping fuel like Tom said. Could be a pump problem and a carb problem at the same time.

Just a note, with leaded fuel a tan colour on the plug is about right but with unleaded you have to look down inside near the base of the centre electrode to see the slight tan residue. The main part of the electrode will appear white.