You are correct that it will be the same for several years. All 350, 403, 455 will work from 68-77. The bellhousing will work from all BOP of the same years. You can check the classifieds on OLDS.com
http://www.442.com/class/oldsads.html Run a wanted ad on Craigslist. They are out there and the firebirds of the 70's era had the most of the 4spd manual trans. I found a guy parting out a mid-70's firebird and bought one that I needed. Be careful that you stay 68-77 as the 64-67 olds cranks had a different bolt pattern.