i did the a-body manual disc conversion on a b-body. i ended up with a 1.03 master cylinder after trying a smaller .93. the car stops but not very well. to do it over again i'd never use a large single piston caliper. beware that some kits use a 2.74 piston for b and e bodies. i had to get rid of the 2.74 and go down to the 2.47 to help in stopping. i have another b-body with 11" drums and power assist and i'd never ever ever ever take those drums off for a disc conversion knowing what i know now. there is absolutely no comparison in stopping.
Did you use a manual brake brake pedal and mount the master with the proper bracket or proper set of mounting holes on the firewall?
A manual brake pedal has a different mounting bracket, and has a different hole for the master cylinder rod to bolt to. That gives the manual brake pedal more leverage then a power brake pedal has. If you used a power brake pedal in a manual brake system, you can't apply as much pressure to the manual brake system that it needs. If you mounted the manual master on the same bracket a power booster mounts to, or in the same set of holes in the firewall the booster mounted to, you have compounded the incorrect pedal problem. There are two sets of holes (or indentions for a set of holes) in the firewall to mount either a bracket the booster, or to bolt the master to, the manual master goes in the upper set of holes on the firewall, and the master cylinder rod bolts to upper hole on the pedal.
I've done probably a dozen of these conversions over the years, never had a problem with the disc brakes being worse then the drums were. Gene