I was going to do the same to mine, with a new mirror. The base is larger on the Pinto, and the aftermarket replacements won't work.
My 78 came with a day/night mirror but the reflective coating was deteriorating and driving me nuts so I put a panoramic rear view clip on over it, works for me.
I have yet to find a day/night mirror that will fit the base of the Pinto.
Talk to the guys at speedy glass, they have a clip on "bezel" that will go over the original base, and you can glue the new base onto the old one so there isn't a gap between the mirror mount and the windshield.