To answer your question....you can simply poke an ice pick or tiny screw driver into the center of the plug and pry it out.. piece of cake. I've heard of putting a propane torch to it and I'm 100% sure that would work perfectly and not do any damage to the base (note....PROPANE torch), but yeah, it would work. However, i don't think the front/rear idle mixture screws are your problem....not to metion if you have factory carbs, they're probably frozen in place.

Anyway, as you figured, you're running lean when you floor it. You have the black outboard springs (stiffest I think or maybe 2nd stiffest). Lean knock is too much air vs. too little fuel. With those stiff outboard carb springs "it ain't too much air": unless you've got a nasty vacuum leak somewhere (check). So, that leaves too little fuel. Check the squirt on squirter. Check to see if the accelerator pump cam is worn. See if you can adjust for a bigger/longer squirt with an accelerator pump cam change (or position change.... position 1 vs position 2).

Fatten up that accelerator pump squirt and let us know if it still bogs.