305 SBC miss fire?

hi guys maybe you can help me out?
i have a 77 nova with a 305 4v automatic, it had a exhaust system that went from single into the muffler and out dual to each side of the car. Up until i changed out the exhaust to dual headers it seemed to run fine, now it sounds like it's missing on a cylinder, but when i checked the headers for heat all were equal. I check the plugs and all are nice and light brown, i also checked the timing which is ok. at idle it sort of chugs by sound only, meaning that you don't feel it when you are sitting in the car. I have checked all the plug wires as well. It doesn't or should i say hasn't back fired at all, just has a very annoying way of sounding, sort of like a school bus that runs on gas sort of chugging.
anyone got any ideas before i start pulling it apart?

thanks
IMnuts

Check for resistance on the cap/wires. May be a weak fire on the cylinder. Also look for vacuum leaks. Its possible that when you had the exhaust changed, you cracked a head when the factory bolts came out. This should be identifiable by a Compression test. If these dont turn up any results, pull the heads.

thanks guys for the good suggestions. So far i have checked for header leaks and found none i have checked for any external vacuum leaks and found none as well. I have check all my plugs to make sure they are gapped right and all are a nice brown, i hate to say it but i'm thinking either the header tubes could either be the prob as they aren't exactly the same length or even a bad cam lobe. Any other ideas i couild check. When it's idling the engine has no shake to it at all at least not like if i pull a plug wire, and the exhuast sounds like a miss fire constantly but not like a popping sound more like a feathered popping almost as if there was an exhaust leak, and it's suck back sort of, if you know what i mean :(

thanks
IMnuts

Have you run a compression test?

no compression test as of yet, was going to dive into it on the weekend when i have more time