Matt, the distribuitor is the same on a 77 as a 80 and the only difference is its either a duel or single vaccume advance. The dual vacuum distributors used the port closest to the distributor body to retard the timing. I forget the reasons why (I believe it was for emissions reasons), but I wouldn't use one unless you're trying to do a concourse restoration. They don't work well. If the engine starts to overheat the thermal sensor (vacuum tree) opens the port to the back side to retard the timing until engine reaches proper temp. The vacuum lines run through a thermostatic vacuum tree that locks out vacuum advance until the engine's water temperature comes up to about 160 degrees. Another emissions boondoggle.And if you develop a vacuum diaphragm leak on one of these dual vacuum distributors you can experience some really weird timing problems until you finally figure out what happened.