On the surface, a project like this would seem simple. In fact it is not.
It is fairly easy to add braking to the fronts but the original rear brakes are done like that in order to reduce the likeliness of locking up the rears.
Adding any kind of braking to the rear opens up what the front to rear balancing should be.
The original rear brakes are like go-cart sized and that was done for a reason.
Good luck on finding the right balance. I'm still looking for it. You really need to rent a race track for a few days if not a week and bring your team of mechanics with you to get them balanced.
Even the pad compound effects this. Not just the pressure the calipers are receiving. It really opens a can of worms.
I will point out that there are more then a few race cars that have the rear adjustable proportioning valve installed in the cabin next to the driver. That's because the track conditions vary along with the tire grip and thus the braking balances.
For a $10,000, production car in 1970, the original decisions on the brakes was understandable. Now the perimeters have been expanded with extensive choices and possibly lethal mistakes. 
You simply can not have the rear out brake the fronts. That could be lethal.