I got a replacement glass for a chevy like an 86 or so I think. It fit good you cant really tell its just slightly a different shape. I know the chevy part makes the trucks value go down but I dont plan on selling lol...

 

Being you're replacing the mirror anyway..........IF you do any kind of towing may I suggest a complete replacement mirror head built for towing?
I just put a set on my '92.
Have a look here:
http://www.cipausa.com/
Click on the l/h menu for "products" then click RV & Towing Catalog.
They are the ones at the bottom r/h corner of page 2.
They have a set screw where the mirror head meets the arm and allows the wirror to extend out approx 6"
I know they're listed on the site for Chev/GMC but do fit the 1st gen's perfectly.
If you need a pic let me know.