Disc brake conversion master cylinder question.

Since that is a new aftermarket part... first is to check that its piston was machined for the manual brake pedal rods rubber retainer. We can't take anything for granted.
Do your 11 inch drum brakes have larger bore wheel cylinders than standard 10 inch brakes? That master cylinder bore might be about right. I have used the info/specs shown at rockauto to sort the differences.