They are different, but only at the bracket mounting boss.
Measurement below shows the distance off the spindle flat to the upper caliper bracket bolt of a disc spindle. It's 1 9/16".
This spindle is the same 69-70 Impala or 69-82 Corvette.
A 69-70 Impala drum spindle has a taller mounting boss. I suspect it could be machined down and tapped to the 1 9/16" seen above, and you'd have yourself a disc spindle.
You can see that tall drum boss on the right image. Pre 69 small bearing drum spindle shown.