Thanks for the reply MT. I changed the fuel filter today... hopefully I haven't disturbed the delicate balance of gunk that's been keeping it going this long
After priming it again a few times the key-on engine-off reading was 54psi. Running at idle was 49psi. Unfortunately the hose on my pressure gauge isn't long enough to get it out from under the hood so I don't have a running reading. The leakdown isn't as instantaneous as it was before; now it leaks down to 20psi in 10 seconds and 5psi in 1 minute.
I still can't find a place to pinch off the return line... the lines are steel up until I lose sight of them down the firewall and by the time I pick them up again at the back of the transmission there are three lines. I think I could make an adaptor to hook the pressure gauge right to the front of the fuel filter; would that give me a good idea of the direct pump pressure?