Stang dreamer:
I've looked into this. There are numerous threads on this. Swapping in a '03/'04 Eaton requires all the intake parts, intercooler and plumbing, and the entire front accessory drive and timing cover to be swapped. If you buy the parts new (if you can find them), you're looking at about $7500, not including the Eaton. Obviously, you can get the parts used, and do a lot better, but it's still gonna be expensive. For your '98, you'll also have to swap heads to the newer PI heads (aka C heads) for the Eaton to fit. Then there's the wiring harness mods for the extra sensors, etc. You can use the '98 ECU, but will have to have it tuned. If you use the '03 ECU, you will have to replace the dash because the '03 will have a digital odometer.
Do a search, and you will find a (small) number of people who have done this, some with full write-ups. I recall one guy in Colorado that built his aluminum Terminator motor and went first class on the build with upgraded internals. I think it cost him about $14k.
If you are determined to have an Eaton, you would probably be better off finding a wrecked '03/'04 Cobra and swapping the parts that you need or swapping the motor complete. If you swap the motor, it's good for 500-550HP with bolt-ons, whereas your '98 shouldn't be pushed past 400HP with stock internals. Maybe you can find one with a good T56 transmission and/or IRS. That's the route I would consider.
Regards,
John.
