I'm new to the forum and recently purchased a 1990 F-250 regular cab long bed 4x4 with the 5.0L engine and automatic transmission. I got the truck for cheap with a knocking noise. I discovered one badly worn cylinder shredding its piston. The block had already been bored 30 over so I opted to just throw a junk yard motor in. I replaced all of the gaskets (except head gaskets) and installed new plugs, wires, cap, rotor, timing chain and air filter. The truck runs pretty good but I'm only averaging 6mpg and most of it is cruising at 65mph. I double checked the timing and it is dead on at 10 degrees before TDC. The truck is totally stock including wheels and tires. I did an alignment thinking maybe the front tires were fighting each other causing extra resistance but there was no change. There really isn't any tuning to do with electronically controlled fuel injection but I really want to get at least 10mpg. Does anyone have any other ideas?

If the centrifugal advance of the timing is stuck, this will cause low highway milage. Mark your timing mark with white paint, chock the wheels. Look at the timing idling, then have someone blip the throttle at least to 2500 rpm. The timing mark should blip down the pulley an inch and a half or so. If it stays put, rebuild your distributor.
Vacuum leaks will cause the carb or MAP if injected, to think that the throttle is wide open and dump extra with the power valve on the carb or injectors on FI. Replace all your vacuum hoses with EPA rated emmisions hose, also probably the PVC tees. Get some binder clips and pinch off the vacuum hoses to the ait-to-cat solenoid, the EGR solenoid, and the hose running through the firewall to all the vacuum solenoids on the climate control system. The run 100 miles and see if your milage improves. If so, remove the binder clips one at a time until you find the rubber diaphragm that is leaking, and replace it. The vapor return hose on your fuel tank can leak vacuum to, if you drop the tank to replace it replace the other two hoses tool
A plugged catalytic converter can also cause bad highway mileage. The only simple way to check is is with a non-contact thermometer, after an hour on the highway, chock the wheels, measure the cat temp and the pipe in front and in back. The cat should be hotter. If not, it is defective.

You might also want to check the fuel pressure regulator. I have seen many go bad between 97000 and 117000 miles. Over pressurized injectors will cause a rich condition.