aren`t the 98 great
i have not replaced my pump yet .i too bought a walbro pump and we have the stupid basket setup for the fuel pump in the tank
i could not get mine to fit so i sent it back.
i beleive the problem you are getting is that with the pressure so high the engine is flooding with fuel, it`s like you have 24lb injectors thats how much fuel your trying to burn even tho your stock at 19lb and your maf is only getting a signal for 19lb injectors .you need to get the pressure down even if you have to buy the pump that ford made specifically for the 98 in the frpp catalog. thats what i`m getting, 255 is alot for a normaly aspirated engine 190 is more than a nuff. as for the voltage i`m not realy sure ask hammer he has the same model year as us.
the pressure at idle with vacum should be 20-22 psi and 40+ with no vacum attached.
how much did you pay for your fuel rail mines is welded to and i need to replace it.
ps. if you ever get the chance do the head swap it`s well worth it