Honestly if I had the time and the inclination I probably could have taken
@Searay205 's suggestion and used a pressure washer to try to blast everything out of the water ports in the manifolds and risers to see if that could clear them enough. It did not appear that the manifolds were rotted so much as the ports were just REALLY clogged up. The question I guess is what was it that was clogging them? Was it built up scale or was it bits of iron (from the engine core) wrapped in scale. I reused almost all of my temp and O2 sensors (one 02 sensor was replaced) and brass plumbing fittings. All in with MA taxes and shipping it cost $7K for all the parts (4 manifolds, 4 risers and respective gaskets). I did NOT shop hard for price, I was looking for availability. I did reuse the CATS and all the original bolts. They didn't look great but they tested fine. For the cost of a $45 riser gasket, I'm TOTALLY cool waiting for the CATS to fail. CATS are about $2900 EACH to replace. One local shop said they would do a 100% replacement of the exhaust system (including all 4 CATS) for $24K PLUS labor which they estimated to be a WEEK ($7K). Took me and my buddy about 8 hours to do the whole thing (and we slow because we were learning). One could say I saved about $24K but I'm sure the shop would have used more new parts so it's not apples to apples.
