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

Valleyant said
No 340 car was equipped with a 7 1/4

This ^^^^^ being the case, that looks like an 8 1/4 with the bigger axle tubes.

7 1/4s have dinky axle tubes.

8 1/4 are desirable rear axles, can be built with sure-grips too. Check with Dr Diff for parts. Definitely worth some money compared to the 7 1/4.