[68 TT]

Getting the TBI tuned properly and the fuel pressure pushed to the top end of the factory range will make a big difference in efficiency. The higher pressure atomizes the fuel better. A throttle body spacer is a proven power and efficiency gain for TBI motors too. The stock ignition system is quite weak. A good cap & rotor, plug wires and stronger coil will make a nice improvement.

Lifting the truck will lower MPG because it increased the drag due to increased frontal area. Keep it as low as you can if MPG is what you are after.

Gearing and tire size have a huge impact on MPG. Rotating mass eats up efficiency very easily. Every inch in tire diameter dropped my MPG by two full points. With 31's I got 14, 32's I got 12 and with 33's I got 10. With a gearing change this might not have been so bad but it gives you an idea of what it can do to your MPG.