How rare is this and what's it worth?

if you are looking for an "investment", dont. it's almost never profitable because of the expense of "NOS Parts" or "factory matching numbers parts" etc you will go broke before you even do the body / paint.

for a quick buck find a 68-70 charger or a 70-74 'cuda somewhere in a barn and then you will break even or make a little profit.

leave this little gem for an A-Body lover who will fix it up just to enjoy. money will be lost but worth it to most of us here.