It wouldn't be stiff, the pedal would get softer.
When you increase the slave piston area (calipers) while keeping the master cylinder area the same, the pedal effort decreases due to an increase in mechanical advantage.
I assume you have t-bird rear disks? If so the reason for the stiff pedal is not running the proper cobra booster which would give you more braking assist with the rear disks. Underboosting it would give a firmer pedal and require more force.
Changing to 73mm fronts may require increasing MC size to the 1 1/16" MC, however if you keep the stock booster, you might be fine with the 1" MC and 73's. You kinda gotta try it and see how it feels. But you are in the ballpark. Just might require some tweaking