mustangluv14, its actually a very simple install. i have a 98 also and just replaced mine due to a crack towards the rear, but i choose to go with a PI intake manifold for the added power. its very easy, and to make things even easier you can take off the hood so you have a little more movement and dont hit your head so much, thats what i did. you will need:
New intake manifold
New intake manifold gaskets (get them, you will probably need them if yours are in the same shape that mine were. mine had about 140k miles)
fuel line disconnect kit (autozone, $5)
some simple things to make your life easier are:
take off the hood
take off plug wires and drape them over the front of the car (make sure to take note of where they go, ie which cylinder)
take off the alternator (i know, doesnt seem easy, but it is. take off the belt, uplug the alternatory, and on the bottom of the alternator you will see 2 bolts. loosen them up, you dont have to take them off as the alternator has "cuts" that will let it slide out.) this will make taking out the manifold A LOT easier.
DONT TAKE THE INJECTORS off the fuel rail, instead take the fuel rails off with all 8 injectors still plugged into it. they literally just pop off, but becareful as gas will spill.
i think this is it, other than make sure you torque the intake maifold to specs, which i think are around 15-22 ft-lbs, but double check just to be sure.
(.:. Brothers Tune-Up and Specification Guide: 4.6L Torque Specificaions .:.
throttle body and plenum are easy to remove, you dont have to take off the throttle linkage, but it makes it easier if you do, that way you can set aside the throttle body.
also, as mentioned above, take plenty of pics if you can, you might not need them, but just in case you do, you can have something to reference. mainly take pics of the vaccum lines, and dont forget to mark them with white-out or something similar. if you need anymore help let me know, i think i still have the pics i took somewhere.