OK 1st post so I will go easy and welcome to the forum.
if there is a big difference in gas mileage between an 79 F-250 and 350 4x4 standard cab
Those 2 truck's would get the same exact mileage depending on if they have the same motor, trans and gear's.
if anyone knew how much better gas mileage an older 7.3L diesel f-250 or 350
What year are you talking about? They did not offer a diesel from the factory in the 73-79 F-series.
so how about a 390 comparison and also a 460
I would think the 460 would use more gas than an FE motor but not a 100% sure.