S code is the correct code for a big block GT Mustang. A code shows for a 289 and K code was the 289 hipo. The X in the lifter valley of a FE/FT block means it's a HD casting. Reinforced crank saddle and main webbing. Some will swear it means high nickle content which is bunk. The X's have been used by Ford on blocks since the model T days FWIW. A C6ME or C6ME-A 390 casting would be correct for your car I believe. So yea there is some difference between the FE block castings. They all look the same but there are subtle differnences. A good 390 block should run around $100. I've been givin more than my share of 390's by guys who just don't want them. I still see FREE 390's every once in a while. I'd look for a complete engine. Buying piece meal costs much more in the long run. You should find complete 390's for $200 or so needing a rebuild and sold as a good rebuildable core.

G.