Your alternator is the external regulator type . Starting in 1973 GM went to the internal regulator alternators . The best thing to do is pick up a power master one wire alternator with the one wire hook up . Then all you need to do is hook up the red battery wire and unplug the the body mounted regulator . And this should work and charge much better then the factory set up did . Or if you have a good 1973 to 1980 Chevy alternator with the plug I can tell you how to make it work on your 68 chevy wiring . I think its better to go with the one wire alternator . I have seen them at some local auto stores . A 70amp is enough for your car .