NEED MOR INFO!!!
 

Come see for yourself. My '95 4x4 Supercab with a 302 and 3.31s breaks over 20 on almost every highway trip I take.
Of course it rarely did before the ignition upgrade.
With the right gears, I've seen other 4x4s get over 20 on the highway.
The 351s use more gas than the 302s but not a lot more. I have seen some smog era 351s that consumed fuel in a very bad way. Don't go to the 351 looking for better economy. If you can afford to lose a little mileage and want the extra torque, the 351 is a good choice.

* CAN SOME ONE TELL ME WHAT KIND OF CHANGES I NEED TO DO TO GET GODD GAS ECONOMY ? ELECTRIC / ECT...

First things first. How many miles on your truck and how does it run overall? Does the performance suffer from any obvious problems?

Your '88 engine is not much different from my '95. The main factors in this case should be the condition, driving styles, gear ratios, and weight/drag factors.

What rear end ratio do you have?