I run mine closer to the springs than that on my street car. The slicks on the 64 have about 1/4" to the frame rail. No issues with rubbing. with either setup. Street car has been this way since the 90's.This was my eye balling...
8" width 4 1/2" back spacing. Looks like all kinds of room.
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Mounted 275/60/15 Coopers. Too tight to the leaf springs.
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After using 1/4" spacers. Should have used 4 1/4" back space.
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It looked like the 4 1/2" back spacing was going to work before I mounted the tires. There's charts out there that will tell you the overall tire width on a certain width rim. Wish I knew that. The 4 1/4" back space would have been perfect.
Are you using the factory axle housing? The '62 - '63 are the narrowest housing among the B bodies. If it's been changed to anything else these specs will be different.
Doug