There has to be some way of kicking the starter drive out. About the only other way to do it is how Brand "F" manages it; they use a big iron blob hooked up to the drive that hangs off to the side of the motor and a set of contacts that shorts out the armature, such that the motor won't turn until the blob gets sucked in enough to break the contacts and un-short the armature.
Lose the Delco starter altogether and get something better. End the problem the right way.
I have a CVR mini-starter; it completely solved all problems I ever had with my starter, even ones I didn't know I had.
"So many Mustangs, so little time..."