I had a similar, but not as extreme problem when I did my combo. I spoke with Lidio Iacobelli at Alternative Auto Performance regarding this. He spoke like this is actually pretty common with the stock computer calibration on the '89-93 5.0L HOs. We went through everything on the phone just to make sure that I wasn't missing something, made some suggestions, and I was able to "limp" it over across state to his shop. He kept it a week, did a full series of cold-start, cruise, WOT remapping with a wide-band O2 sensor, dyno, laptop, etc.
That said, your situation sounds like there is something wrong, versus a simple computer cal issue.
Your pump and PSI sound more than enough for the job...in fact, too much. However, as RPM goes up, voltage to the pump may drop, resulting in less and less fuel flow...just what you don't want. I'm not saying that is your problem, but something to consider. I have a Jacobs Electronics 14v "Accuvolt" voltage regulator that literally increases and locks the output voltage at 14v. Others have used a Kenne Bell "Boost-a-Pump". Along those lines, check to make sure the alternator is putting out an appropriate amount of voltage, as well, as that may be the source of low voltage.
I am assuming there are no "check engine" lights, or else you would have mentioned that.
Another thing, which I am embarrassed to mention, is that when I first put the engine together, I put in the distributor off one tooth. Easy enough mistake. The car started right up, idled fine, but it was running so lean that the headers would glow within a few minutes. I was baffled by this until the fine folks on StangNet pointed out what a Doofus I was.
Sure enough, pulled the distributor and adjusted it correctly, and everything was fine.
New combos may be a bit finicky, and I did have some issues with poor idle quality, baseline idle settings, erratic firing, and the lean issue. A fair amount of tuning later and some professional help, and all is well.
