You can bolt the early housing onto the late manifold.  I had Jeff R.
make me a little adapter plate to make it work as there is a very thin
area where the two surfaces barely overlap if you don't use it.  If you
use the early (Chrysler) thermostat you could make the adapter plate
yourself with a drill press and hole saw, I wanted to get fancy and have
him cut a groove in it so I could use the Chevy style thermostat that
you can get at every FLAPS.  Just make yourself a thin steel or aluminum
plate with a hole in the middle the same size as the hole in the late
neck, then drill the 4 bolt pattern for the early neck, set the Chrysler
style thermostat above the plate when you assemble.

nate