6AQ5 is the audio output tube, so presumably about 5K primary
impedance and 8 ohm secondary, although I'm not picky on these
parameters.

What I am picky about is that the transformer mounting holes are on 2"
centers. Looking at the AES catalog I see 2.375" centers, and also
1.5" centers, but the 1.5" one sounds really really too dinky.

Any other places I should be looking? Cannibalizing an old AA5? Don't
know the impedance parameters of those but they can't be too far off,
can they?

AA5s usually used 50L6s or 50C5s, both of which use 2.5K output
transformers. Secondaries usually feeding 3.2 ohm speakers, so you
might be able to get by if you use an 8 ohm speaker. However, you will
take a significant loss of bass (low audio frequencies). But in a
communications radio, that may not be that important.

I could suggest the use of an L shaped metal bracket to mount the
transformer to, and to mount to the chassis, but that would look rather
ugly. You probably are concerned about the looks of the chassis, so
that's not really a viable option.