Yeah... I'm getting the point where I'm waiting for the punch-line.
Narrow band O2 sensors are dumb creatures.
They switch between:
off-scale high
off-scale low
unswitched
Too much time in either mode, throws a code.
No signal, throws a code.
So, either this car is running rich as eff and the EEC is disregarding the inputs (arbitrarily on its own) or gas is mysteriously returning to the mother ship.
I mean... I've seen poorly matched combos suffer mileage issues but this seems to be pretty extreme.
Do you have a trunk full of turbos or nitrous oxide bottles or something getting you to 1000 HP? LoL
When I first installed my Kenne Bell onto the (then) new stroker combo, it ran. 42 lb injectors and a MAF calibrated for 42s. It ran like

actually. I drove it over the Allegheny mountains and got [horrible] fuel mileage but I mean, I could definitely tell it wasn't happy.
A tune brought the whole thing around. Fuel mileage --and-- drivability were drastically improved.
Your story though:
"I got no codes, it runs and drives great, I'm not leaking anything, it doesn't stumble, but if I rev it between gas stations, I'm going to need a tow."
I paraphrased.
Makes no sense
Out of curiosity, would you describe your fuel system from tank to injector?