OP you have very limited options if you don't (can't) run a tuner. You will only gain a few HP out of a tuner anyway.
Unlike the popular belief on RV.net, gears don't make power. If you don't like to rev your engine to peak HP it does not make any difference how many gears you have. If you want peak power, you have to rev your engine to peak RPM. It's that simple.
If you want power at a lower RPM you only have a few choices. Buy a diesel (which you say you don't want to do) or buy some sort of a supercharger. Whether that be a turbo or belt drive.
Another option you can also use is if you want more power is E85. I think your Ford can use it? If so, you should be able to gain on the order of 25 to 30HP buy switching to E85. Be aware if you switch to E85 you will go through fuel at an alarming rate but you will make some more horse power.
My suggestion is just let the engine rev and do what it needs to do.