primaries

I have a 13 GT that I'm looking to buy long tubes for. I'm looking to go with Kooks LT's w/ the catted x-pipe. I do have a couple questions 1) I'm hung up on whether to go with the 1-3/4" or the 1-7/8" primaries. 2) I'm looking into lowering the car on Ford racing K springs (1.5" drop); will...