When I first got my truck, it would overheat going up long grades maybe 1/4 mile long. On level ground it would run fine. Went through the typical cooling system replacements; radiator, water pump, tranny cooler, etc. with no changes. I had installed a new header and brought it to my friend's muffler for a dual exhaust install. When he cut the pipe off the front of the cat, you could look into it and see a black, gummy tar-like substance covering the cat honeycomb. I don't see how it ran at all. Finished up the exhaust, replaced the cat with a resonator, no more overheating and it ran great.
I've never seen a procedure for flow-testing an exhaust system in the FSM. :dunno: Does your truck get hot climbing up long grades? I'd bring it to a muffler shop, have them cut off the cat inlet pipe, and take a peek inside. Replace the cat if there is any doubt.
EDIT: Another thought: were any modifications made to the engine during the rebuild? Like an HO head on your Renix block causing exhaust port blockage?