I have a 1978 F100 Ranger XLT, it have a 302 cid and I would like to know if it is possible to change the engine from 8 cylinders to 4 for reasons of weight and power, my pick-up weights 5512 lbs.
The 302 produces about 135hp and 243 lb-ft and give about 7 mpg

 

Possible, but you'd be money ahead to buy a newer truck by the time you get all the parts you'd need.

You should be doing better than 7 mpg.

Serving suggestion: Rebuild that 302 and add an OD transmission. You could easily double your HP, bump torque and lower revs at highway speeds and you should be able to make 15mpg out of that, give or take depending on where you drive.