All I am finding on the GM herritage link is 6 inch rims with a 61 inch tread width. https://www.gmheritagecenter.com/do...formation-kits/Cadillac/1960_Cadillac_VVI.pdf If that doesn't work open this click on archives, click on vesicle information kits and then click on Cadillac. https://www.gmheritagecenter.com/ I'm thinking your 3=1/2 inches is close. Some years of Cads had wheel with the outer lip shaped a bit different to let the Cad hub cap hook up. I used to hate them things in the 70's when I was installing tires and doing brakes on them.
What you got on there now? Start with something and see if you need more or less. If your in a pinch go 3.5 backspace because with a spacer you can get to 3”. If you order 3” you can’t get 3.5 out of it.
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