a friend of mine is putting disc brakes on his 69 roadrunner, will the front drums (10") from a 69 B body fit on my 71 duster or are the spindles a different diameter.

If no ans (on here) might check one of the online part houses (rockauto?) & see if both of those apps have the same part # for a front drum

And that runner should have had 11 inch drums, which would be gigantic on an A body.

I can't say or recommend you do it but the RR drums will not fit on your A body spindles , you would have to use the B body spindle which has a different upper and more than likely lower ball joint than your duster .

If you treat it just like the popular A body disc swap, then yes it will fit.
In simple terms, you WILL need the upper control arm from a 73-76 DISC brake A body car because all B body cars use the larger upper ball joint. ALSO, the 73-76 A body cars with 10 inch front drums also used the larger upper ball joint and control arm.
If you use the appropriate upper control arm along with the B body knuckle, drum, brake hose and lower ball joints, everything will bolt up. I did this exact swap on a 76 Dart to gain the larger bolt pattern. I replaced the left hand thread lugs though.