most states, yes, that is illegal. pa used to be that it had to exit rear of the passenger compartment and/or rear axle, but it was changed now to it requiring to exit at the edge of the body. as for back pressue, no muffler will not hurt it if the cat and stock manifolds are in place. if you really like the sound, i still suggest the muffler eliminator thats sold on ebay for about $30

 

yes the cats are there but they were gutted about 7 years ago. would that be an issue?