I ran across a youtube video that might help. Fast forward to about the 5 min. mark.
https://www.youtube.com/watch?v=wjo01V_8j1s
And from this forum, the 73-87 Message Board, do a search for blower motor. Numerous threads will come up. Just read through them and you will most likely find out the info you need to help with your repair.
Hope this helps!! And Welcome.