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

If the calipers are making contact with the wheels, try a pair of these:
Set of 4 | 3mm Hubcentric Wheel Spacers | 5x100 w/ 57.1mm Hub bore | Flat | eBay
Generally not a fan of wheel spacers, but at only 1/8" thick it's more of a shim than a spacer- you won't need longer studs, and should be plenty to gain the necessary clearance without having a major effect on clearances elsewhere.