Newbie Alternator and Belt Change Instructions

No fear. Changing a slant six "fan belt" is one of the easiest jobs. As mentioned, you must remove the p.s. belt which is harder, and best to change it too since belts cost ~$5 on rockauto. You should see how tough it is to loosen the alternator on my 1980's M-B cars (designed by drunk Germans). Keep a spare belt in the trunk. For roadside emergency, you could cut off the p.s. belt and leave it off (p.s. is only needed in parking lots). In emergency, main thing is to keep the water pump and fan spinning, which you can do w/ nylon rope. In an old car, you can drive ~200 miles off the battery if no headlights.

I would use the later square-back alternator like you have, since it gives more output, esp. at idle. You simply need to ground one field terminal, which it appears was done in your old one. You might also change to an electronic voltage regulator for 1966 (rockauto). Many posts.