[1968 Pontiac]

I haven't tried it on my truck, but you might try the brackets from a 9C1 (police package) Caprice. They came in two flavours:

a small adapter arm that attaches to the small alternator/ps bracket
an alternator/ps bracket specifically made for the large alternators

I have both set-ups although I'm currently using the adapter arm on my 91 Caprice. What I did find was that one of the large alternators I have, which is the one that came with the bracket specifically made for the large alternator, will not work with the adapter arm set-up as the alternator has too large of a diameter so it fouls at the section above the ps pump and can't be dropped enough to be bolted to the arm. All of my large 9C1 alternators are Delco's, so the problem isn't a cheaply cast housing made out of tolerance.