‘71 340 Duster Front Drum to Disk with Factory 14” Rallye Wheels - Problems

Confirmed 4” backspace…..

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I just noticed this, but, you're not measuring that backspace correctly. Backspace is measured from the outside lip to the wheel mounting surface, not from the shoulder where the tire mounts. The backspace on those wheels is probably 4.25".

How wide are those rims? Are you sure they're factory? Because I don't think any of the factory 14" SBP rallye's would have 4" of backspace, let alone 4.25". The widest factory SBP rallye's would be 14x5.5, and they wouldn't have a 4" backspace.
I am assuming that going to a 15” wheel would also solve this problem completly. Would these wheels from Wheel Vintiques have adequate clearance?

15x6 Chrysler Rallye Silver Small | 5x4" bolt | 4" backspace​


A 15" wheel should solve the issue based on where your contact is, yes.