i was looking for some bigger headers for my 3.0l and instead of spending $200 on brand new ones i thought i could swap some from a 4.0l since they might be bigger because they are from a bigger engine
Would this work and mount up to my stock cat system?

 

No. Cant do that.

The exhaust manifolds on these trucks flow very well. You wont see any gains unless you are planning on forced induction.