There is a difference...look at them closely. The 258 "narrow" mount has different notching in it but can be modified to work.
The V8 (304-360-401) will vibrate more than the 258...Remember that. The notch in the 258 mount will allow for "MORE" vibration when used on a V8.
People have used a thicker "rubber" on the 258 mount to work on the V8.
I've just always wondered why...? Why?
Those small block 8's that AMC made are in a lot of other cars if not scrapped...Javelin, Matadore (was an option), and the AMX.
I actually saw several of each of these cars in a yard along 81 up towards RC some years ago. I needed a motor, but they were gone beyond needing a total rebuild.