Got the rebuild finished today. Broke two bolts rebuilding the output shaft assembly, one doing the valve body transfer plate, and had to fab a custom one for a stripped oil pan hole that the PO of my new transmission had just stuffed full of RTV. Was doing the air check for the rear clutches wrong, unnecessarily tore apart the pump assembly to get back down to the drums, and nearly stripped one of the pump bolt holes in the process. Fingers crossed that one doesn't decide to leak, but at least it's closest to another hole, being one of the two next to the breather.
Wrestled with the park rod for what seemed like forever, but at least for now the new transmission is buttoned up and ready for installation. Couldn't find the right fittings yet for installing the temp gauge into the drain plug slot, but I think I can order something online and just swap it in when I can find the time to drain it and add the fittings.
Started tearing everything out for the swap this afternoon; removed the big skid plate, the driveshafts, the starter, and the transmission mount and got everything unplugged and unbolted from the tcase and trans. All that's left are the four torque converter bolts, the bellhousing bolts, and the cooler lines. Should be pulling the old unit tomorrow, cleaning and sourcing some miscellaneous parts this week, and the new one will go in Friday/Saturday if I can't find the time after work before then. That'll give me two weeks for testing, break-in, and the likely possibility of needing to take something apart at least once.
Doing a new transmission mount, new valve cover (unrelated but it was leaking pretty bad down the block and making everything nasty), and possibly a new starter. Flywheel looks good, and other than a whole lot of rusty bolts to replace, should be good on parts. Fingers crossed everything goes somewhat well. I still have to figure out what I'm going to do with the old transmission. Probably just sell it as a core or trans that needs to be rebuilt; I don't think I'd want to chew up my limited summer wrenching time rebuilding a unit to sell.