wrote:

What about the opposite conversion. Retro-fitting a Hassy 1600F with a P6 mount?

No go. You wouldn't be able to focus it to infitity. P6 has the shortest
lens registration distance of MF SLR systems (which probably explains
why the viewfinder image is so undersized - there isn't enough room for
a full-size mirror). http://www.a1.nl/phomepag/markerink/mounts.htm
says:

Hasselblad/Kiev88 multi start thread 82.10
Hasselblad 500/2000 bayonet 74.90
Pentacon 6 breech lock 74.10

With newer Hassies only 0.8mm = 1/32" of metal to grind away. That just
might be feasible, if you can solve the shutter problem.

-- Lassi