I helped my hunting buddy replace the manifolds on his '07 Armada last year. I think he might have sprayed the manifold nuts for a week at most. They all came off OK. But some of that may have to do with where you live (road salt, etc).
With LT headers you're going to need to be tuned and you'll need new b-pipes that have the sensor bung after the catalytic converter. Cajun B-pipes can take care of all your exhaust needs.
I was trying to PM nightowl regarding a service engine soon light with a catless set up but i need 15 posts to be able to do that. I guess i'll have to go make some dumb comments to get that number up. lol