Woog - This web catalog page from LMC for an Auxiliary Overload Helper Spring (pictured below) is what popped up in my initial search on the web.

The 2nd pic below is one I found at 67-72chevytrucks.com board.

From what I can see, these being mounted on the frame rail itself should not cause any interference with spare tire.
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)