Jeff,

I can't answer directly as I haven't done the retro.  However, I'm
looking at a partially complete kit with an eye toward changing out the
tail.  I look forward to hearing the educated responses to your
challenge.

Anyway, my MK II tail fuselage scalloping was 99% eliminated with my
belt sander to get the correct camber on the fuselage.  I then added the
requisite bid schedule (I believe it was 6 plys) to the fuselage sides
building a new scallop with the correct camber.  Subsequently the tail
was hysol'd on with a very small amount of flox.  In regards to
strength, (I'm no smart guy), I think all the tail loads are down
forces.  Thus putting on that huge bid schedule on both fuselage sides
below the tail and additional bids right down the middle provide
abundant strength (I'm sure abundant means a lot to a physics guy,
sorry).

I suppose the real issue here is take off the vertical tail or make some
nice access panels in the fuselage sides just below the tail to allow
access to your work.  I'd go the access panel route.

Let us see the pic's after you sort it out.

Larry