My GT5 engine came with a triple prong (autolite I thnk) plug and I've had terrible 4 stroking with it. My attempts to remedy this include, first going to a smaller carb jet. I went from the stock 70 jet on my RT carb to a 60 and it helped a bit but not anywhere near eliminating the 4 stroking. Next I tried an old NGK B6L plug I had laying around and that helped a bit more. I did some research on here to find out what plug folks think works the best and found many saying the NGK B6HS was the way to go so I ordered one of those. That plug didn't seem to perform any different than the B6L. Then I read someone saying the BP version will get the tip down lower to be in a better position to ignite. I could only find the BP6HS-10 and I'm not sure what the -10 means, but in any case I have almost completely eliminated the 4 stroking. I should add, my engine is still in break in so I'm running a 16:1 mix for now. Even at 16:1 it is amazing the change I've experienced.
Change your plug; change your life.