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.