Howdy sir.. The side tire mount was connected to the top of the bed with 2 feet and had 2 other little feet connect to the fender. I think. The fender was cut out in that top left rounded part to make a place for the tire. From what I can remember that is what it looked like. I have not seen one up close in 25 years. I think it was made of 1/4 round rod one end flattened and had hole in it where the bolt held it to the bed and fender.
Here is a pic of one, I am not sure if you can see the makeup of the mount well enough to tell. I would bet any of the old Chevy trucks with the step side bed and the spare tire mount on the side of the bed would be pretty much the same.
http://www.oldcartrader.com/ocdetail60188.htm I hope it was OK to post that link, it was the only pic I could find.
The other place to keep the spare is under the back of the truck, just like most all pickups are today.