Either one will work, you'll just have one unused groove with the double.
What you DO need to do is to mount the alternator correctly so the grooves will line up, and not stress-crack your alternator (or brackets) again by mounting it crooked.
Look at this picture:
View attachment 1716207749
The alternator mounting tabs go to the OUTSIDE of the bracket.
And it looks like you've got a cobbled AC inner alternator bracket instead of the correct spacer used on the non-AC setup.
So do yourself a favor and get the correct mounts along with a new alternator.
Since you're using an aluminum water pump, you'll need the '70 and up brackets.