Same as my previous post -- use 79-83 AMC Concord/Spirit/Pacer brakes. No modifications necessary, just swap everything from the backing plate on the drums and from the caliper bracket on the discs out.
Better yet, go to
www.scarebird.com and order their AMC disc brake kit. It will work with ALL AMCs. Scarebird just makes a bracket and seal adapter. They send you a complete list of other, common auto parts store parts to complete the setup along with the original application and part numbers. You buy new GM hoses and calipers, Ford Ranger rotors and seal. Along with the cost of the Scarebird kit you end up spending just under $400 for brand new front disc brakes.
The only thing is you need to have a 1" bore master cylinder. The 66 with drum brakes has the right one. Remove the residual pressure valve from the front brake outlet of the master cylinder and you're all set! Do this by screwing a small screw 2-3 turns into the little hole in the center of the outlet. Pull straight out on the screw with a pair of pliers and the tubing seat will come out. There is a small spring and plastic (or hard rubber) piece behind the tubing seat. Remove those then push the tubing seat back in and you're all set. If you have problems with the rear brakes locking up during hard stops on wet or otherwise slick pavement, you will want to install a proportioning valve in the rear brake line. You probably won't notice it on that heavy a car (Ambassador) unless you really have to slam on the brakes hard (panic stop).
Don't worry about the reservoirs of the master cylinder being the same size. Older disc brake MCs have a larger one for the front discs so you don't have to check the fluid often -- really not at all. If you check fluid level at least once a year you're fine with the drum MC in this case. Engineers wanted to idiot proof the fluid level, so they made the front one larger to ensure you have brakes even if you never check it. Newer models often use same size reservoirs.