Shifting into reverse would either blow the tranny and disconnect the engine, overrev the motor to destruction, or blow the tranny and the engine simultaneously. Only one way to find out for sure!
My friend in high school did this in his '69 cougar convertible. The entire drivetrain locked up and the engine died, but he was able to start it and drive away.all the other gears
A week later the linkage fell out of it and it went forward in reverse, reverse was gone and all the other gears appeared to be neutrals.