I can't comment on the 2600 Spider without knowing the year other than to say my old 64 2600 Sprint had disc brakes in the back.
The 60 Spider and 64 SS are very likely different. The 60 definately has the small drum with straight fins and steel shoe bases. The 64 SS may have the very late helically finned drums with aluminum shoe bases- they essentially look like the 1960 two leading shoe front drums, though I'm not sure if there are any common parts. If you have both to look at the difference will be obvious.
I once put rear drum components from a 59 Giulietta Sprint onto a 62 Giulia Spider without a problem.