340 Chugging instead of Reving

[1] Plugs should be NGK 5 heat range. Colder will foul like you have now.
[2] Unless the carb has been adjusted, I would expect a considerable amount of transfer slot showing below the pri throttle blades at idle....unless bypass air has been added. Too much slot showing causes a rich condition......& plug fouling.

Thank you for the plug rrecommendation. I started with an Autolite plug that I found recommended with the Magnum heads.

2- I'll nend to ask my carb builder buddy the questions regarding the carb. He rebuilt it when I bought it used and made adjustments during this time of searching for why the engine was acting this way. He's been building Holleys for late modle and modified asphalt cars for 20+ years. Luckily I've known him since 5th grade so he comes as a.bargin lol