75 is when the subframe made the most changes due to the cats needed for emissions
a 74 subframe should work without issue, here is a thread on a different website outlining some differences http://transamcountry.com/community/index.php?topic=68213.0
The main difference that you'll run into is when getting control arms as the earlier subframe had smaller holes, while the newer ones have larger holes. Just make sure you're buying suspension parts for a 74 camaro instead of a 70-73. If you have existing parts you can modify what you already have with a drill.