I have what I believe is a 1927 Stewart Warner model 500. Can someone advise what the resistance should approximately be for the interstage transformers?

Oh, say around 1K on the primary and 3K on the secondary. These transformers for various TRF sets are regularly replaced with Stancor A-53 look-alikes by Hammond - available from RadioDaze, tubesandmore, et. al. The typical turns ratio is 1:3 in a voltage step-up mode.