There is a guy on the SDC forum that has been asking for some time now
about the differences in a Hawk power drum and power disk/drum brakes.  
I won't go into all the problems he has (since he is trying to do
something that is not possible without a lot of $$), but since both of
my Hawks have power disk brakes I cannot fully answer the part of his
question about the following:

1) Pedal length is it really different between Power Drum and Power Disk/Drum?

2) The differences between the master cylinders between Power Drum and
Power Disk/Drum.  I replaced one of mine several years ago and the
person at SASCO said that the one for disk/drum did not have a check
valve and the other one does.  I checked the service manual and it says
"All 63/64 Hawks"  had master cylinders with check valves and I don't
have a real strong desire to pull mine off to see if it does or
doesn't, nor do I know if it really matters.

3) The differences between the hydrovac units.  The power Drum unit has
a long snout and has a bleeder on the end, the power disk/drum has a
short snout and no bleeder.  Other than that I have no idea what the
differences are?  Has anyone tried swapping these to see if they will
work?

Thanks