I had the reverse situation with my 68. The previous owner had converted it from 3spd manual to auto and just took the clutch pedal out, leaving the narrow brake pedal. I wanted the wider automatic style brake pedal. I ordered a repop pedal twice from different suppliers. I never did get the right pedal. There is a slight difference in the 68 dash and pedal assembly that makes it year specific. I had to take one of the repops and rework it to make it right. Both repop pedals were listed for a 68 but were not right.
I'm not saying that your situation won't work, just a heads-up that the 68 is different than all other years. You may have to just try it and then modify accordingly. The repop manual pedals for the 68 might be right. Good Luck!