Welcome to the forum
That is correct. The '71 and '72 12 bolt will be the only direct bolt in differential to your truck that will have the 5 x 5" wheel stud pattern. But the odds of finding one with the gear ratio you want
and a posi is going to be very difficult and $$$$$ when you do. The '73 and up will require cutting and welding to install.
Of course you could always install different gears and the conversion axles in your current 12 bolt. A taller tire will help lower the rpm's with your current gear ratio. Have you considered an OD trans?