Good read here: Rebuilding The Ford 4.0L Pushrod V6, Doug Anderson, Automotive Rebuilder, April 2001
Also here: The Ford Ranger 4.0L Engine - The Ranger Station
4.0l OHV was in Rangers from 1990 to 2000, in 2001 the 4.0l OHV was discontinued and Ford switch Rangers to 4.0l SOHC, these two engines are not related.
4.0l was a very reliable engine, my '94 has over 300k on bottom end, I made the mistake of over heating it once, and cracked a head, so...........if it starts to overheat, DO NOT try to make it home, or anywhere else, lol, let it cool off and drive it short distances or get it towed, the tow will cost less than a new head.
It makes 160HP but better torque at 220ft/lb than many V6 engines
There are not many upgrades for this engine, if you want more power in your Ranger then a 5.0l V8 is the way to go