New headers... i'm starting to get nervous

Hello guys, I have a bit of a situation going on, and I hope i can get some friendly advice on what to do.

On my k2500, the stock exhaust manifold on the passenger side of the block has a crack all the way around it about 1 inch from where it bolts onto the head. It is the port closest to the radiator. I have noticed that the crack gap changes as they get hot and cold etc.. I can also hear a pretty decent exhaust leak driving down the road. A mechanic told me that i am probably losing a decent amount of horse power because of the crack, and that i should replace at least the one manifold.

I started doing my homework on replacing these manifolds, and got a massive wave of options that consumed most of my time for the better part of a week. Because my truck is a TBI, getting headers with an O2 sensor hole is a really good idea from what i can tell. I also like the idea of these OE shorty headers. They supposedly give you 15-20 hp and better torque at lower rps, witch if true, would be icing on the cake.

I ended up getting these headers here:
http://www.summitracing.com/parts/SUM-G9014/

I got these mainly because it said that it is a OE replacement part, and comes with everything i need to install them. And from what i can tell, it seems like it will be an easy change over. Just unbolt the old manifolds, and bolt on the new headers, per the instructions using the gasket etc...

The main reason i am doing this is to solve my exhaust leak, so any tips on keeping my install of the new headers as sealed up as I can would be great.

Also, my truck as this smog stuff in it that i don't mind removing completely. The new headers are not compatible with the A.I.R system (smog stuff) and i'm not sure how to go about removing the smog stuff. I unbolted a black plastic tube thing, that was attached to a metal bracket that has a hole about the size of a quarter that pumps out air/exhaust back into the system to be mixed with the intake air (assuming? The idea behind the smog stuff, is to re-burn the exhaust right?)

Can i simply plate off everything that the A.I.R is attacked to and be good to go? If it is not the easy, what's the solution to this?

I am (plan to shortly) removing the stock manifolds, and the A.I.R system, and replacing the manifolds with aftermarket OE headers that do not support an A.I.R system. Is there anything I haven't mentioned that might be an issue with this?

Thanks guys.