Phil, skip those back to back electrolytics!
Use a nice Mylar or Polypropylene cap. Looks like you call for ~10ufd, these are easy to
find in over 400volt ratings...

Since ur driving  7k ohm primary, do you actually need or want 10ufd?
A queeck calculation says that 10ufd and a 7kohm load gets a -3dB frequency of
 2.2747952684 Hz!
So 1 ufd will get 22.27479...Hz.
Which, imho is still to low, given the inherent core limitations of the mod iron!
Something between that 1ufd, and ~0.5ufd would in theory put the -3dB around
30-40Hz, which is pretty darn good.(if it works as the calculation holds, then 0.6ufd will get
38Hz as the -3dB point, which BTW will still have a fair amount of energy an octave below!)
 And, will help avoid inadvertently swamping the mod iron.

Not to mention that the driver iron isn't going down to 2Hz or 20Hz... fwiw.

Testing in practice to find the actual -3DB point is a good plan.
For easy testing, half voltage is 6dB down.

                              _-_-bear