How much horsepower should I expect?

You did say that you have the carb and headers already. I say use what you have and see how it runs. Spend some time dialing in the advance curve and timing and carburetor. I think your combo sounds pretty good and is enough to destroy the tires at will. The bigger parts some are recommending would add more power but the carb will cost plenty of money, the long tube headers will cause you headachs as you already know and if you run what you have first you will have a baseline to compare any changes you make. I've put bigger and supposedly better parts on some of my cars in the past and lost power, so if I had'nt had the smaller parts in the first place, how would I know? Use what you have.