The power increase wouldn't be worth the effort, IMO.
But if replacing the engine anyway then it might be worth it.
4.0l SOHC has 207 HP and 238 Torque numbers
5.0l has 215 HP and 288 torque
The older Ranger 4.0l OHV was 160 HP and 220 torque, these were worth the effort.
Good read here on the V8 swap in '98 and up Rangers: Ford Ranger/Bronco II 2.9 liter Engines