I ended up going with replacement springs from Eaton Detroit Spring Co. in Detroit, MI. They have been in business since 1937 and say they have over 24,000 original manufacturer spring blueprints. Mike Eaton was a good guy to work with on the GTO springs.
It was interesting, their online ordering form had a leaf pack with five leaves listed; I let Mike know that was a mistake. The leaf pack from Pontiac for my GTO had six leaves in each rear leaf pack and I sent Mike the pictures of it. Mike worked with me to get the correct six leaf pack for my GTO and updated his website with the correct number of leaves in the leaf pack. (I can only assume that he didn't get too many orders for replacement '74 GTO springs.) I ordered the stock spec replacement springs and they raised my car's rear by about an inch, back up to the factory level, which is what I was looking for. You can also order the springs from Eaton to be higher or lower than stock specs.
Once I got the original springs off and compared them to the new springs from Eaton, I could see how the old springs had broken down and flattened in the centers. I replaced the shocks on all four wheels at the same time with factory spec shocks. Once everything was in place, the ride was improved and the stance was back to the factory look. The overall effort wasn't too bad, but then again, my GTO has been garage kept since day one, so basically rust free.
I took some pictures of the replacement process and plan to put together a short photo-accompanied description of the process I used to replace the rear leaf springs when I get some time this winter.