My 74 'Cuda is a BS car & it is not a slapstick(don't ask how I found out) now it could have been changed over the years, thats for sure!
Your car would have been. All E-body console shift Auto cars used the Slap-Stik shifters.
Maybe you lockout piece broke off? Do you have the return spring?
Shift, shift and into Drive and no further. Push button to gain access to [N], [R] & [P].
Like Chryco said give it a clean and lube it up and it will work like new.
If that doesn't work install your new shifter(s) set up from eBay.