Not sure what is wrong with the brakes on my 1968 Chevy C10

[HeavyHauler]

The brake pedal on my 1968 Chevy is very,very,very mushy and I can push the pedal all the way to the floor with very little effort and it doesnt stop the truck until you floor it.

It Feels like there is still air in the system although I have bled the master cylinder twice while off the truck and I tried bleeding the system with no success. I then took the truck to a local shop and they couldnt fix the issue either.

Here are the modifications that I have made to the brake system on my 1968 Chevy C10 fleetside short bed. I converted it to 4 wheel disc brakes. All parts are brand new. The truck originally had 4 wheel drums.

*Front disc brake kit from Early Classics
*Tuff Stuff 9 inch booster TFF-2224NB
*Tuff Stuff brake booster rod and clevis kit #4750 (replaced stock rod)
*Master cylinder for 1975 Corvette
*Summit Racing GM prop valve for 4wheel disc system #760185
*Rear disc brake kit from Captain Fab that uses 76-78 Cadillac Eldorado calipers with built in e brake levers and rotors from 92-02 Chevy Astro AWD van

I am getting fluid from all four calipers,bench bled master cylinder twice and bled system numerous times with no success but each time it seemed like the system didnt have any more air in it as I wasnt getting bubbles anymore.

Again also took the truck to a local shop which I dont think the tech was very knowledgeable and he was not able to solve the issue either.

Any suggestions on what might be causing he issue?