i have an 84 CJ7, 258, AT. i have a bad crack in my exhaust manifold and want to put a header on it. the only other performance mod is a Weber carb. i want to know what header/muffler setup would benefit me the most. would dual exhaust benefit me? what size in/out on the muffler? what size exhaust pipe? any advice would be helpful. thanks

Dual exhaust won't help you on a straight 6. Any decent header with 2-1/2" pipe, a high flow cat and a free flowing muffler will work. I use a glass-pack. It's not like that 258 is pushing enough air to require any more than that. If you're looking for more performance put fuel injection on it.