Yeah, not having overdrive I don't see that you will get much better than 10-12, depending on what gear ratio you are running.

I have a '94 F250 4x4 with a 351, C6, 4.10s, and 32s. It gets around 13 to 15 mpg on the highway. Usually right about 14. With a 460 you are going to do a little worse, probably 10-12 mpg. My biggest recomendation would be to slow down, esspecially if you have 4.10s. Try driving 55 mph instead of 65 or 70. My truck gets 14 at 55 but it goes down to about 11 or 12 at 60 and around 10 between 65 and 70.