what's a original 7 1/4 duster rear end worth

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Match up the rear cover bolt pattern. Good Chance the 8 1/4 has the 3.21 ratio.

340 Duster open diff ???
Good chance it could have a sure grip too.
Don't get rid of it before you pull the rear cover and check that ring gear and diff.

Tags on the rear cover bolts can identify it too.