It isn't possible to rebuild the 94/95 heads with the 96/97 valve guides. You would have to replace the heads if you want to solve the valve guide issue. At that point, you might as well do a PI head swap on your existing bottom end. I certainly wouldn't pull a motor just to do the head swap, but if you have other issues like a leaking oil pan gasket or rear main seal, then pulling the motor to do the heads would make sense, and as Maic mentioned, swapping to a complete PI motor may be more cost effective than the head swap by the time you buy heads, gaskets, head bolts, new timing components, etc. Any PI motor will work, but the Explorer ones use an aluminum block instead of cast iron, so they save about 75lbs off the front of the car, which makes a noticable improvement in handling.
Having said all that, what makes you think you have a bad head gasket? On a 4.6, a bad head gasket is extremely extremely rare, as in being a mechanic and working on Fords for the last 15 years, I have never actually seen a 4.6 2V blown head gasket. Before condemning the head gaskets, and thereby possibly condemning the whole motor, make sure it isn't the oil filter adapter gasket, the water pump, the intake gaskets, or the heater tube that goes under the intake manifold. All of those are fairly common coolant leak points on the 4.6, but without even seeing your car, I would bet money that the head gaskets are not blown.
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad