GEars, what gears are you running and what tire size?
Also cooling is important. A 180 degree thermostat may help you run cooler. Gas mileage may suffer slightly, and the Apten chip may need to be aware of the changes.
A stock thermostat runs the truck at about 210-215 degrees with a in tune cooling system and a 185 stat the OHV will run at 190-195.
remember the thermostat only controls the minimum tempature.
Also for a street driven truck there are some electric fans available that can free up some HP for you.
It seems that for off road trucks with the OHV no electric fan is adequate for all situations, meaning hillclimbs, mud etc and most people run into overheating problems. But there are many people out there running electric fans on the street with no issues.
Also dont forget about important things, such as feeding your K&N nice cold air, not hot underhood air.
Good plugs and wires are mandatory for a strong running OHV, I prefer Bosch +4's but some people disagree. I also like NGK and Motocraft plat.
Taylor makes a great wire for the OHV as does Jacobs.
Intake porting/gasket matching is something yu can do yourself for the most part and free up some power. the lower intake on the IOHV can use some help, the uppers flow pretty well stock but can always use smoothing.
Where you will see a vast improvement in OHV performance $ for $ is with a CAM. The 410 grind of 422 grind , (410 for a DD is recommended). This can really WAKE UP a alreadyy modded OHV 4.0L, the stock cam is a dog.
The best thing to do after all this, is have the heads done.
Other things to consider are:
full synthetics, trans temp and fluid (auto), Clutch type (5 speed), fuel pump and injector upgrade along with the MAS, a larger cat (I run a 2.5 cat on my OHV) and I personally like the headers on the OHV, but the amount of work required to install them is usually the reason why people pass them up, butif you are going to do the work yourself (dont mind), have the engine out, they are worth the $$$