I have installed several of the under floor conversion kits. I discovered that the large pivot pin that connects to the pedal and the linkage was oversize and would bind up. I subsequently used the original pin and everything worked OK. I'm not sure this is your problem but it is a possibility.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine