Dana60 question

Apparently the owner of this 1973 Duster put a Dana60 in it before he sold it to me. At least, I think it is. I got under the car and the rear cover looks just like the pics that I've seen of others on the web. Is there any other way to be sure?

Also, it also apparently has some REALLY short gears in it. Like 2.73 or so. I'd like to get some 3.90s in there if possible. Where do you guys suggest going for those? Summit?