Soooo, what is the best plane to build around a corvair engine?

Not to avoid your question, but ...

There is a lot to like about the Corvair engine(s), and also some things not
to like. I really like the idea of the 6 cylinder smoothness. However,
like the VW, you are usually starting with a very old engine. From time to
time, I have heard rumors that heads and crankcases are still made for
Corvair engines; but I have never been able to locate a source, and for me
that is sufficient discouragement. In addition, the engines were not
originally developed to turn a propeller cantelevered on the end of the
crankshaft--although there may be "workarounds" similar to the drive Steve
Wittman used or his Formula-Vee racer or to the outwardly similar "rear
drive" for the WV derived Great Plains engines. And, if you choose that
route, you might also need to design a bed-mount...

Anyway, the some of the real Corvair affectionados probably congregate
around William Wynne's web site: http://www.flycorvair.com/ and there
appears to be some discussion of airframes. There were a number of designs
back when the Corvairs were still fairly common (as cars), but the only one
that I personally recall is the KR-2. Plans are still sold, but it is only
a 2 seater if all the occupants light and of small stature. So, like all of
the homebuilts, either find one to sit in or else build a cabin mock-up
including seats and cushions before you build anything else!

I hope this helps.

Peter