I have just put your calculations through Feldons calculator. I just want to make sure that I have the correct information, You are making a cylinder cooking chamber 48" long and 24" in diameter? Therefore your cooking chamber will be 21703.68 cubic inches,(and your firebox should be 1/3 of the cooking chamber size) so you will need a firebox 7234.56 cubic inches, or the nearest I can get to it is 19.5" x 19.5" x 19.5" which is 7234.56 cubic inches, and not 24" x 24" x 24".
The feldons calc that I am using does not specify a specific chimney pipe diameter size, you just add your own diameter pipe, and the calculator lets you know what length it has to be, so for a 4 inch pipe cut to 28.76 inches, and a 5" pipe it is cut to 18.41"
I am using this calculator: