It is doubtful a 2004 Henschen axle will drop into place on a 1966
frame. Henschen changed the mount bracket design over the years.
Doubtful the shock mounts will be the same either. Modifications can be
done with a big drill (11/16") and a torch and welder though.
http://vintageairstream.com/Myrtle/axle.html
Andy at InLandRV.com is the only source for the axles...
Best of luck,
RJ
VintageAirstream.com