Hey guys, im having a 383 stroker built right now around the 500hp+ range and im using a set of GM fastburn heads. Im guessing i should go with the 1.75" primaries with that amount of hp or will 1 5/8" cut it? Are there any other headers that will work with the D port design besides SLP? I cant see forking out $700 for a set of overpriced headers and i don't really want to run LT headers. What are my options?

Difinitely 1-3/4" at least. Your engine has more than enough breathing capacity to make good use of 1-3/4" headers if its makes 500hp-ish.

Is SLP the only company that makes the 1 3/4" shorties for our cars? Would i have to run duels if i used longtubes? Or could i run a 4" single back with LT's? Where can i find the mufflex systems, seems like nobody has them?