for a 98 its not as easy to recalibrate the needles but.. I can help you with this
First off the easy ones. Start the car and stick the battery and the oil pressure right in the middle as thats where they will be (the oil pressure gauge really isnt a gauge on 96 up stangs....)
For the temp gauge wait until you are 100% sure its fully warm and stick it right in the middle as well (it sucks as well just make sure the car is warm)
For the gas gauge go fill up 100% and stick it on the F or a touch past F actually (right on F to be safe, generally its a bit past F when all full, you should know where its at so just stick it where it is when full)
that takes care of all but the rpm and speedo. You will need a friend for this one just to make sure. Google the rpm calibration vs speed ratio for your rear end gears and transmission. You have 2.73 rear gears and an AODE mustang transmission. Find the rpms at 55mph for your vehicle.
Get a friend with an accurate speedometer to go 55 mph and you pace him with cruise control and very carefully ease the needle on. Have him slow down to 45 then go 65 and make sure its all accurate and your good. Now stick the rpm on at the correct position for 55mph then check it at 65 and 45 and your done.
If you had a 99 up you could do this by yourself as it has a digial read out
but oh well... I know it sucks but its how you have to do it.
For other tips do NOT push the needles down all the way they have to sit up a bit so they dont get stuck on the gauge face so make sure they are not all the way pushed down. You can plug the gauges up with the plastic cover off of them so you can put the needles on when the car is moving, just be very careful not to bend the little metal pins and take your time to get them JUST right then pull over and snug them down and double check.