When I installed Wilwood calipers on my 65 Corvette, I had a problem getting a proper pedal. It would go to the floor after bleeding everything.  It turned out the caliper pistons were stuck in their bores. (From being new, not that there was anything wrong. They just were not engaging with the pressures available after bleeding).  The fix was simple.   I just stomped hard repeatedly on the pedal to apply enough pressure to move the pistons.  All was good after doing that.  I only mention this since you are using Wilwood calipers.

I think your 1-1/8” master cylinder will be too large and not develop proper pressure to operate the brakes. Foxbody master cylinders have two different size pistons in them and are typically avoided. A 1” bore tandem master cylinder like the 92 Ranger disc/drum unit would be more suitable.

The only thing as far as another suggestion is that you have a defective booster.  The MustangSteve booster system is designed to use the single diaphragm 9” (four cylinder engine) booster (but the v8 booster such as yours can be used).
Typically if a pedal acts on its own, the booster is defective.

Also, and this is typically never a problem, be sure your brake pedal can return to its upper static position without its travel being limited by a rubber stop or anything else.