Just a FYI, most all aftermarket heads will be drilled for the 1/2" head bolts. I was able to find a couple of sets of decent aftermarket heads for only $400/pr used (those 5.0 Mustang guys get rid of all sorts of good parts). Both sets are cast iron - one set of TFS high ports, and the other was a set of professionally-ported World Windsor Seniors.
If you want to stick with OEM ford heads, grab a set of GT40s off a 96-97 Explorer 5.0. Or the late 97-2001 Explorer5.0 GT40Ps. Either of those are much better than your current (or any other) factory castings. The GT40s have 64cc chambers, and the GT40Ps have 59cc chambers. Either can be found in your local wrecking yards for relatively cheap (<$200) if you don't mind doing the work to get them off.