I have to push gas pedal to floor to gain speed

Sounds like you lost you timing belt.

I’m happy to assist further over the phone at https://www.6ya.com/expert/howard_977676954994c5b2

who puts a 4 cyl in a mustang??? 4 cyl cant push that kind of weight,i have a briggs and stratton motor that might work better

Not quite everything is fine.
Is the check engine light on ?
When you push on the gas pedal, does the engine speed change ?
Can you rev up the engine in park ?
What have you checked so far ?

Have your exhaust checked for restriction. Sounds alot like a clogged catalytic converter.

Fuel filter stopped up

The gas pedal is recalled on many fords and toyotas and is a well known problem, get the pedal replaced

Could be clogged cat converter or it could be timing out of adj.

I'm a big chevy fan and the reason it wont shift is because of the big tires.What happens when you put big tires on it and still have the same gear ratio the tires slow down the speed alot.There for making the trans seem its still going slower.The only real adjustments you can do here are first adjust the kick down cable if it has one,Next would be change the gear ratio to compinsate for the big tires.But if you have the elctronic trans theres really nothing you can do to adjust the kick down.But if it does this cable controls how the truck shifts on where the gas pedal is.To adjust the kick down cable you need to look at the cables that go up to the throttle likage.the cable thats on the bottom is the one you need.Next behind the bracket that holds the cable is a metal tab its actually in the cable.Push that tab in and the center part of the cable which looks like a con push that all the way in release the tab.Next jump in the truck and press the gas pedal to the floor now its adjusted.Thats all you can do..Hope this helps you and have a great day..

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589

2 things...either the timing is off or more than likely the catalytic converter is plugged...How many miles are on it??

sounds like the clutch might be sliping to test try pulling away with handbrke on if the engine races without any pulling against the brakes the clutch is sliping and you will probely notice a burning smell