Fuel pressures are very different between the return and returnless systems. Return system is variable between 2 and 2.5 bar (29 - 35 psi), the returnless is fixed at 4 bar (58 psi).
Trying to get the PCM to work with the wrong pressure is tough without a custom tune and even then, you've have to rework at least some of the fuel piping anyway.
Worse, your '98 expects the fuel pressure regulator to be engine mounted, the '01 5.0 expects the fuel pressure regulator to be in the fuel tank so your current mix has no pressure regulation at all.
Think the easy solution is to do a bit of u-pull-it breakers yard diving and get a returnless system from a later Sport or, the full EFI system from a late '97 (no PATS) or '98 Expo/Mountaineer V8 and use your current low pressure system. That would keep all the bits OEM Ford.
Or, an after market, external regulator/return
Don't forget, even the "returnless" system has a return, the 2nd fuel line at the back of the fuel filter is the return going back to the in-tank regulator.
The '96 and early '97 Expo use a different EGR valve and monitoring system so while not an impossible solution, not as simple as the later '97 or '98 systems.