Spiff,
The original Atlas drawbar that I have is 8-5/16" long from the end of the threaded end to the face of the thrust bearing (which sits against the left end of the spindle). One end of the rod is threaded 3/8"-16 UNC by about 3/4". The other end has a 2-1/2" dia. handwheel (which are available many places). Total length of the rod depends upon how long the thrust bearing is and how much needs to be up in whatever handle or handwheel you put on it.
The thrust bearing (I would use steel instead of aluminum) is 1-1/2" of 1-1/4" dia. solid round, drilled through 3/8". On one end is a 0.767" dia. pilot turned about 1/8" long. This keeps the rod centered in the spindle so that it pulls straight on the collets or cutter holders. The pilot works better than a taper as it keeps the rod centered even when you loosen the drawbar several turns and bump it with your left hand to free the collet.
Whether or not you use 3/8" rod and thread one end or use All-thread, I would put a handwheel on it. Any diameter between 2" and 4" should work fine.As you get older, you'll appreciate having the handwheel for the added torque multiplier it gives you.
Robert D.