Further testing done:

1981 non-Turbo gas: I've been having some problems with my car stumbling and dying. I thought it was related to problems with the evaporative emissions system because a gas station guy had force-filled the tank until it was overflowing, and the problem started immediately thereafter. However, the venting system looks good, and now I think the problem might be elsewhere. Art has been halping me through some tests, but that thread is getting crazily long, so I'm starting a new one with some tests I did today.

Voltage tests done at idle, warm, unloaded:

Across battery terminals: 12.7 volts.

At fuel pump relay spade #87: 12.14 volts.

At pump side of connection under back seat: 11.9 volts.

Results are the same using a spare fuel pump relay.

Fuel delivery:

Tested at return line connection near fuel filter. System pumps 35 ounces in 30 seconds. I can blow by mouth through the line the rest of the way into the tank with the gas cap removed. The gas I pumped out during the test looked clean.

Any suggestions? Gas delivery seems fine to me. Voltages all seem too low. I don't know what to do next, if these low voltages could cause the stumbling and dying problems I've been having, or how to get the voltages up where they should be. The alternator was tested last week and the guy said it is fine.
--
Thanks everyone for all the information and advice, Doug C. 81 242 Brick Off Blocks, stock, B21F (non-Turbo), M46; 86 244, B230, 140k , auto.