I'm asking so don't think I'm challenging. Why does it need to be progressive? We have had good luck w/the set up on HiHo Silver and it gets wide open throttle. It has a BBK throttle body do you think that is the difference? The BBK should be the same as a stock Mustang as far as the throttle arm and shaft are concerned.
You have to look at a couple things to see why it's benefitial. The explorer throttle body is bigger than the early mustang one. Also the stock Bronco accelerator pedal travel is limited. To make the limited travel work between idle and full throttle, linearly, the throttle body needs to open fast. This is a problem with a big throttle body when you need to be delicate (rockcrawling, etc). You could change the lever ratio to make the opening slower but that would prevent you from achieving full throttle.
The solution to this is to make the throttle opening progressive. Near idle the opening is slowed down to make the engine speed very controllable. Above half throttle the opening is sped up to provide full throttle when needed. It works very well and is almost like having a lower gear near idle. The throttle is very controllable and less like an on/off switch.