You might want to get your heads checked while you have them off. I also thought I had a head gasket problem, but as I searched around on the forum, 1st gen heads are notorious for cracking. I read that someone looking for used ones ended up going through 4 sets of heads before he found 2 good ones.
91-92 (91TM) are the worst, and redesigned heads came out on the 93-94 (93TM). I ended up going ahead and replacing my heads with aftermarket 93TM style heads, that are supposedly thicker than stock heads for $160 ea. Figured by the time I cleaned the old ones, had them checked, with a good possibility of them being bad, might as well just put on new ones, with new vavles, new springs, and new seals.
I used a upper gasket kit /w bolts from NAPA made by Victor Reinz, distributed by DANA (Ford OEM), about $190. Autozone had a Fel-Pro kit for about $150, but I heard bad things about Fel-Pro. The NAPA gasket set matched all the original gaskets, except for the lower intake, and it looked even better.
Gaskets you will need.
Head Gaskets (duh)
Head Bolts (even if reinstalling old heads)
Valve Cover
Lower intake
Upper intake
Intake-Fuel Rail (not req'd, but good time to change it, I didn't)
Exhaust Manifold
Thermostat
I did not originally have exhasut manifold gaskets, but since they came in the gasket kit I used them.
