I would think that to fix the timing on a 4.6, you will need to have control of the computer.. and if you have control of the computer, you can easily set the timing to be ideal.
Just as trinity said, the ideal timing changes with load and rpm. If you lock the timing to something that performs well at WOT, it will be much more sluggish in normal driving. If you set it up for normal driving, it will rattle like crazy under heavy load. Even at full throttle, the ideal timing changes with rpm by as much as 6-8 degrees. Your emissions will be poor, you'll get worse gas mileage and the car won't be as much fun to drive.
In short, don't do it. You'll make the car run crappily. And again... WHY would you want to do that?