the cat on all early gm trucks were bolted on. if its about 2 1/2 - 3 ft long and about 2 ft wide, then its probable the factory cat. or atleast replaced with afactory one, dosent sound like it though. i drive an 87 with 223,000 plus miles on it and i took my cat off about 4 years ago. i havent had any probs with it yet, and if you have any type of aftermarket muffler on it it will be considerably louder. one good way to tell if its cloged is to drive it, if it feels like it just wouldent get out of its own way if it fell off a cliff then it may be cloged.