I would bet most people are running 2.6" piston calipers. All the stock 73-76 A-body calipers are 2.6's. The only way you'd have 2.75's is if you're using B/E or the F/M/J/R calipers. So you'd have to make a point to get those.
Regardless, that shouldn't cause any problems. The pedal feel would be a little different for a 2.6" piston compared to a 2.75", but both would stop the car just fine and neither should hit the floor if everything is working properly.
