I really do not know if the stock mustang master cylinder has the residual valves. Was the stock master cylinder at or below the drums or calipers? If so it probably did. If it is higher than the disc and drums probably not. You might have to consult on old tech manual to get that answer. The speed shop advice is correct, the 2 psi residual valves are for the disc, and the 10 psi are for the drum brakes. They are supposed to be installed as close to the master cylinder as possible. Also when you buy them try to get the ones where the fittings come with the residual valves. I had trouble finding those at the parts houses. To tell front from back trace the path of the brake line coming out of the master cylinder to see which set of brakes (front or rear) it is going to.
Another problem you will have when the master cylinder is near the same level as your drums and calipers is bleading the brakes. When you do this, connect your bleader hose as usual, bring your catch jar above the level of the caliper or drum, and pump the pedal until there is no air. You will not be able to gravity blead, and the other usual method doesn't work to well either.
Hope this helps.