Help with my car! latest

since i have installed the racing muffler steel headers and twin carbs the car seems to get no high end power, i can shift from the top of third and instantly be in the top of fourth, not sure if it has something to do with the distributor cylinder or air fuel tuning but i cant figure out whats wrong for the life of me, If anyone could fix it and give an explanation of how they fixed it/how they determined what the issue was that would be great!! Thanks guys.