I think the spacers are more to adjust the angle for clearance with either the fender, the hood, or the engine components.
For dragging brakes the cleaner method would be to either adjust or modify the brake rods (there's at least two that are important here) to relieve any pressure on the master cylinder's piston.
The other mention of spacers is usually between the master and the booster to accomplish the same thing as shortening the rod(s).
The pedal-to-cantilever rod is sometimes adjustable. Sometimes it's a cut-to-fit in the original kit due to there being so many slight differences between one Bronco and another.
The booster-to-master rod is almost always adjustable to a certain degree, but there are different length rods too, depending on which booster is used.
Ether way though, there should be at least a few thousandths clearance (I think the spec is like .050" or so?) between the rod tip and the back of the piston.
So when mounting the master cylinder, if there was tension pushing on the piston before the body of the master bottomed on the booster flange, then the rod needs to be shortened. Or the master is wrong and has too shallow a piston recess for that particular booster.
The pedal rod is perhaps less finicky, but it still can't have any pressure against it when the bracket is bolted to the firewall. If it does, things could go wonky in a hurry.
Did you just install this Paul? Did you buy a kit or piece it together? Was the kit new or used?
Paul