I've been following this thread with some interest and thought I'd put in my rare $.02 worth. Seems to me that combining SPA/BPA or anything else for that matter is simply a matter of loosely defining the allowable airframe/engine combinations (much like modern F3A is defined). The vintage of the plane would make no difference.

And this leads to something that is missing in both SPA and BPA. While we're wrapped up in the airframes of old, we're missing the creative element of the old days. There is no provision for original designs. Wasn't that part of the past also? Limiting building materials and techniques to balsa would also encourage new affordable designs (no high tech materials).

This way any era plane can be entered, to include new and original designs, at an affordable price.

I'm probably full of it though, so feel free to ignor this post if you wish.