OK. I believe I have done the best I could within my limited scope of knowledge and things are OK so far. I had to skip the nitrogen pressure test because I just couldn't find an economical way to pull it off but the rest seems to have fallen in place:
0. replaced compressor, expansion valve, and ripped hose. Thoroughly flushed system and dried with compressed air.
1. added the new drier last thing and immediately pulled the vacuum.
2. left system in state of vacuum (pump off) several days)
3. ran pump again for 30 min or an hour
4. added freon (R12)
5. did my best to look for leaks with soapy water after the system was full of freon. I didn't find any. (If I had found a leak I'm not sure what I would have done since the R12 was already in the system. :-))

I was confused as I was adding freon because the high pressure gauge seemed stuck at 100psi as I was adding freon. Turned out that the high pressure valve seems to have malfunctioned? It was a new valve but I can't get either the low pressure or high pressure gauge to read anything from it all now. I guess that must have happened when the high pressure side reached around 100 psi. Not sure why that happened.

Also, it seems like the cans don't really empty unless I shake them or periodically turn them. I know I'm not supposed to do that but I did anyway since it seemed the only way to empty the cans.

The low pressure side seems to vary between 20 and 40. 40 when idling. 20 when revving.

vent temp was 42*F today on a 90+F Wichita day so I guess everything is working reasonably OK for now. I was expecting colder in the vent but maybe that's reasonable? The compressor seems reasonably quiet. On to the next set of problems I guess: (leaking tranny and oil, odo is intermittent.)

The whole site-glass bubble thing was pretty confusing too. I don't see any bubbles now when the system is operating so I guess it's either full or overfull.

It all ended up being more of a mystery than I had guessed it would be. The high pressure valve malfunction really threw me.

BTW I also had some problems getting the compressor to kick in without jumping it. That was an entire journey to itself, ending with me discovering that the OVP relay in my car is bad. I've jumped that for now and will order a replacement this week.

Thanks to everybody for the help and advice. This forum is amazing! :-)