I would look at your brake pedal ratio. Recently I switched from drums to front disc on a 68 Lemans. Original brake were power brakes with 4 wheel drum and with new disc on the front the brake pedal was very hard to push. Honestly I thought it was low vacuum to brake booster . I made some changes and added an electric vacuum pump. The vacuum pump really changed little. I did not think to check the pedal ratio, however when I did it did not look right, the pedal had two positions for the rod from the brake booster one hole for manual brakes and one for power brakes. I used a formula and recalculated a better pedal ratio. Once I made the new hole and reinstalled the pedal the brake pedal feel was greatly improved and had an excellent feel and response.
If you check the ratio and drill a new hole for the pushrod and Clovis from the booster I would recommend the new hole centered to the width of the pedal.
Here is a simple diagram to calculate the pedal ratio, hope this helps.
Jim