Disc Brake conversion issue
As far as I know, the only "small bore" truck master cylinder is a 15/16" unit for a Dakota application, which definitely won't fit your A-body manual brake push-rod.
Did you install the rubber push-rod retainer?
Is the push-rod FULLY seated in the piston?
If you unbolt the master cylinder and it springs forward, the push-rod is pre-loading the piston.