The majority to a doors structural strength at least as far as the early B-bodies is the inner door panels themselves and two cross bridge gussets at the doors inner front hinge areas. I suspect that the other year and make doors are more or less the same except for the safety crash bar doors of 73' and up.
Now as DVW has done, his car is a one purpose car that sees little to none hard usage in opening/closing of doors and functional windows and such, so no worries there. His effort as a race car is the extreme of weightlessness (done beautifully) without adverse impact to that type of usage. Could it be used in such way on the street? maybe, but not without a concerted effort by the driver to be gentle with it and taking into account the grind of your teeth on the integrity of the door in a high usage environment as time goes on.
If you are using doors and windows as in a street car, I would keep the inner door metal panels in place, but gut them strategically so as to keep 'em strong and functioning and give a little teeny, puny, itty-bitty, petite, level of safety in a collision.
Just keep in mind that doors before the mid 70's were relatively light in structure. It is the inner workings of window rooster cranks, window glass,frames, lock mechanisms and even the crank handles/levers themselves that are heavy, not to mention the crash beams.
Crash beams are for just that, crashes. Do they take away from the doors ability to keep shape/form, no. unless of course, a crash like with any other door.