i already have the slp cat back system. but my question is since the slp is just a bolt in type muffler, could you buy a muffler from another company and put it in with the clamps?
i've been wondering for a while so i thought i'd ask

 

Pretty sure no. But i guess if it was another modular catback then might as well try.

 

ya because i see some people selling some used 40series mufflers, that came of mustangs, on craigslist. they usually sell them for like 40 bucks so i was wondering if i could pick some cheap ones up and put it on

 

The SLP mufflers are completely straight, center in/center out, so if you replace them the new mufflers will have to be the same, and even then they might not fit. They'll need to be the same length and same inner/outer diameter as the SLP's on each end, as the SLP's slide inside on one end and go over on the other.
You could just take it a shop and they could probably fit them in for a decent price.