It just depends on what is causing the current condition. I lost both studs that go from the manifold to the catalytic converter a couple of years ago and had the tractor sound for quite some time. It ran awful in the winter. Patched the leak and ran way better. But I now have misfires that I know are coming from the plugs and COP's, while having a small exhaust leak on the other side.
COP's can misfire without throwing a code/CEL, and it is common for it to only misfire under load. From what you describe, I would start with the manifold leak. However be prepared to replace the COP's and plugs next if the problem is not resolved.
Word of advice, pay someone to do the manifolds. We tried to do one side on my dad's 2000 expy ourselves, and it was way more hassle than it was worth. Not to mention you need a welder to get broken studs out of the head. Plugs and COP's are doable by the average joe, as long as you have some patience.