97 F150 engine swap to 97 ranger i4

Alright, I got this kid living with me wants to buy a truck needing a new motor. He wants to do it himself. *I'm helping with my old man who's a mechanic but busy atm.* He wants something more fuel efficient than the stock 4.2L. so I was thinking either a big i4 2.3 or 2.5. only 80 Hp difference between the 2.4 and 4.2. So in regards to this, what is the smallest I can get into a 97 swb single cab F150? And still be cost efficient.

3.0L maybe or cross-manuf. A 3800 series 2 by GM? Just looking for a really big i4 or smallest v6 I can possible get. I know I have to watch out for bellhousing bolt pattern or I'm looking at converter cost and that's not cheap

A 4 cylinder is not going have torque and if you install it you will find that you don't save any fuel since you will have to drive it floored almost all the time. If I had to install a small 4 banger like this I would put 5.13 gears in it and drive it slow. Honestly even a Ford 4.0 from a Ranger is not gonna make you happy.

Alright well I talked to my father, you are right that the 4.2 L is the minimal I can get unless I bored out a 4.0 and that's not cost nor has efficient. We are just going to rebuild the engine.

The 4.0 is not a smaller version of the 4.2. You can't "bore it out" and get more power.