Unfortunately, there isn't a good donor car for a lower gear for you. Very Few cars from your generation ('73-'77) had any kind of gear at all. If you can find a stick non-ac car that would be your best chance for a lower gear but so few were ordered that way when new. How many of those are left that are parting in wrecking yards 30 years later? Another remote option is maybe a 455 GP or Grand Am, most had 3.08's but, again, how many are left on boneyards? You may be better off looking into a new r&p that will fit your rear. I consider myself fortunate that I traded for a 3.42 posi for my '73 GTO. They are hard to find.
Steve
www.pontiacheaven.org