It might be sacrelige to say but leverage is what you want. If you look at curves for gas engines, adding headers, tuners, intakes, etc can only take you so far.
Cam, head porting, etc can increase performance. Those would break the bank.
When I was towing with a Dodge 5.9 gasser - the largest bang for the buck I could get was a K&N intake. It was only a very slight diff but it meant being able to take mountain passes at speed rather than bogging down into 2nd gear.
The next big bang for the buck was rear end gears. About $1200 to do it. Make a two hop change in gearing and you have more leverage to do the work without winding out the motor on the highway.
What rear end gears do you have ? My Ford has super tall highway gears and is a bit of a dog (3.42 I believe). 3.73 would be much better. Don't be tempted to go too far on the gearing.
You are entering an area of chasing the dragon. Tread carefully and don't get bit.