Would there be any benefit in swapping the exhaust manifold on my 91 with the manifolds from a 2000 Cherokee? It could be hooked up to the factory 91 exhaust with a Y-pipe. How about the O2 sensor? Mount that in the Y too?

Just looking to get a few more mpg's out of my Cherokee and willing to spend a bit to do it.

 

I'm pretty sure the manifold on the 00 has two outlets whereas your 91 has only one so you would at least need the rest of the exhaust which I'm not even sure would fit and you would likely end up with an extra cat because of all the emissions shenanigans.