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The 76 and 77 brake systems should be the same. But I would be wary of this sale. Depending on how long the 76 has sat without use it could have deterioted the brake calipers from the inside. The brake fluid could have "gunked up" for lack of better term, and the calipers would have to be split into two halves, pull the pistons out and inspect for debris and contamination. At that point you have to rebuild the calipers with new pistons and seals. You could do the same exact thing with your calipers and end up with the same rebuilt caliper scenario without having to buy a set of used calipers.
Probably the same, but only worth core value if they are standard (stock) calipers. One or more will leak. I would never install used pads or even rotors on my car, unless they are virtually brand new and a cheap price. Make sure that they were never reworked on a brake lathe.