I did it on my '66 Chief and will do it on my '66 Laurentian wagon this summer.....

 First of all, find a donor.. '67-'70....... Pontiac or Chevrolet.

 Unscrew the 3 lines from the valve, unbolt valve from frame, 1/2" wrench, and remove the master, booster and proportioning valve with lines attached. You will need the to snip off the the rear brake line about an inch back of the valve because you need the nut to be installed onto your existing line. Or snip line first and leave it attached to the valve! (different size thread wise than the '66 but the line is the same size)

 Remove your existing splitter by unscrewing all 3 lines, unbolt from the frame. Remove your existing master with splitter attached as a unit. You will have to remove the 2 upper brake pedal bracket nuts and studs from the firewall as the brake booster studs will take its place.

 You will also have to elongate the holes a little for the booster studs to fit so test and file accordingly! Remove the flare nut on the rear brake line and install the donor nut and reflare the line.

 Install the booster and hook uo to the brake pedal. Then install the donar master cylinder and proportioning valve and hook up your lines.

 Bleed the system...............