Fuel filter or fuel pump is a good possibility. If you do get a new pump it has to be 98 specific. The 98 was a transition year and uses a 97-down return style system with a 99-up style pump. Yeah...
Also what plugs did you use, did you gap and torque them properly and did you do new plug wires too? If so which wires? When you did the plugs did you have the MAF exposed at all? Maybe something got on the element? There are a lot of things it could be honestly.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.