I agree with changing plugs, fuel filter, oil. Are you going to do this yourself??? Be prepared because the plugs are not easy the first time that you do them. Use Motorcraft Plugs and if you can afford spring for the Magnecore Wires because they are a perfect fit and great performing wires.
AOD transmissions are notorious for failure. Mine went out at 94,000 miles and was rebuilt. I would consider changing out the tranny fluid and maybe adding a shift kit to improve shifting and help prolong the life of the tranny.
The resonator is the single largest restrictor in the exhaust system. either replace it with a different resonator or have a straight pipe welded in. The mufflers are not too bad but you can do better. I have a straight pipe and two magnaflow SS mufflers and it sounds really good. I get comments on it quite often by even non Super Coupe owners. There are also some lips that can be ground down at the end of the exhaust manifolds on the inside to open them up some also. You will need a die grinder to do that.
Remove the air silencer, get a larger MAF, and maybe a 5% or 10% pulley and you will notice quite an improvement in performance. The smaller pulley will be an improvement that you will be able to feel instantly.
I have a 90SC that has a all of the above plus a 10% and a SC Performance CAI and it runs pretty good. I get a solid 15 lbs. of boost in the upper rpm range with no knocking.
And BTW if the headgaskets have not been replaced that repair will always loom and may happen quicker once you start modifying the car. The boost is actually the amount of back pressure that is measured right before the air goes into the intake manifold. I can assure you that increasing the amount of boost does not help prolong the life of the headgaskets.