87 5.0 - Exhaust in cab..help!

Unless you have some rotted pipes the most likely place for leaks is going to be at your 4 socket/ball connections at either end of the h-pipe. Usually you can tell a leak by the telltale black soot around the leak. If you just put a catback on you may not be able to tell that easy if there is a leak unless it is cold out and you can see the exhaust. If you nail the throttle and is sounds like an old chevy truck then there probably is a leak somewhere. If you can smell it while your driving then you definately have a leak. Normally you only smell it when you are idling and the exhaust can linger and filter up through the car. I would check for any cracks in your floorpan, missing bolts, and your seals around the e-brake cable and shifter housing.