If the master cylinder is a disk type it won't work right AT ALL with the drums in the front. Not to mention the proportioning will likely be wrong if your car originally had disks, which it appears it did from the fender tag.
Buy a set of disk spindles off of e-bay, they usually sell around 100-150 bucks. But try to make sure you get the later ('73 on) spindles so you can buy the later rotors, as they are much more common than the earlier 70-72 2 piece design rotor, and much less expensive. Any spindle off of a '73-'76 A body will work, or a '73-'74 e body. '73 and up B, F, and R body spindles will also work, although they're slightly taller, you'd never notice on a street driver.
I'd find a set of '73 up spindles, get a couple of 11.75 caliper brackets and convert the whole thing to 11.75" front disks. The spindles and caliper brackets can be had on eBay, the rest you can get from Rock Auto, or Summit, or your local parts shop. I'd bet even buying everything else new you could come in under 500 bucks if you did it right...
