We made more progress today. Separated the C4 transmission from the engine. Then stripped off the engine accessories to get ready for some serious degreasing.
One area that was a bit of a shock was the cooling system. Lots of scale and rust in there. And while taking off the 5/8" bypass hose from the thermostat housing, the nipple of the housing just disintegrated, staying in the hose! And where the bypass hose goes into the water pump and intake manifold, the passages were completely clogged. So, yes, thank you to everyone for making me pull the engine/trans. I would have found this issue leaving the engine in the car, but having it out just makes it that much easier to deal with.
I wasn't planning on pulling off the intake manifold, but at this point I really have to in order to see how much scale/rust there is. If there's a lot in the intake runners, what's the best way to remove it? Will a hot tank do it? I'm taking my radiator in for service and they may be able to hot tank my manifold, but I haven't asked yet.
The water pump has a FoMoCo casting. Is this the original water pump or did the dealer service pumps also have FoMoCo? I suppose it's possible the water pump was never replaced, but mileage is over 150k. I know I haven't replaced it, but I really haven't put that many miles on it considering how many years I've owned it.
Below are photos of the C4 transmission with a few factory paint markings. I can't quite read the last one. Some sort of OK stamp maybe?