Guys the prewar frame has a lot more than just the dropouts, the rear fender mounts, the crank height, the shape of the lower chassis bar where the center skirt mounts, the rear rack mounts are not there, the front of the tank area and i also believe the rake on the neck might be angled out slight more, the chain guard mounts. Might as well be a different frame, i've got two '39 Dx's and even then between the two there are differences.
In any case any ratrodder or restorer would trade you a postwar, (1946-1955) Dx frame for your prewar 1939-1942) dx frame. If your gonna build an X1 start with the correct fame at very least.