Okay, I haven't done a lot since last month, but I have done more than nothing. . .
I hooked up the Auto Xray, and got "vehicle not responding" when I attempted to scan. I checked for voltage at the ECM connectors, not knowing exactly what to probe or what to expect (all I have is a general Haynes manual, which doesn't tell diddly about that), verified 12 volts on the orange w/white stripe wires (or was that pink/white? can't remember for sure) and red wires (one type switched, one unswitched). So, I went to Checker and picked up a reman ECM (required my old PROM), installed, didn't start; scanned, "vehicle not responding". So, it sat until tonight.
The '90 SSE set the SES light today, scan said TPS voltage too low, taking a quick look at that I discover the connector is broken at the TPS - but the pins were still making contact. Probe for voltage, get 5 V on A, zero on B & C - again, not knowing exactly what to expect, I went out to the '91 and did the same thing - got 12, 7, and 7, respectively (with ignition on - I didn't think to check it with ignition off). Poked around the harness of the SSE where it goes under the alternator and PS pump, it wasn't routed too cleanly, unwrapped part of it and made sure nothing was pinched or cut, rewrapped it. Put the 91'* TPS on the SSE, set it to .49 VDC at idle, SES light goes away, that'* fixed (runs better than it has since installing the L27, in fact - TPS must have been weak all this time).
Thinking that I may have crushed the same harness on the '91 when I did the alternator swap, checked it, no problems. Cleaner than the SSE, actually.
One other thing I haven't mentioned yet: When cranking the '91, the fuel pump doesn't come on until oil pressure comes up. Before this problem started, you could hear the pump come on for 5-10 seconds as soon as you turned the ignition to "On".
So, any thoughts on what these latest bits of evidence may be trying to tell me? I'm real close to pulling the engine/tranny out of the '91 and putting them in the SSE - it always ran better and the transmission operated more smoothly than the SSE ever did. But, my son would like the '91 to drive if we can figure this out.