They are all pretty much the same, performance wise, after the EGR was added.
The only thing different is the later ('79 and up)? manifolds used a different thermostat and twisted it over to one side to clear the AC compressor. So, if you are looking for more of a stock look, try and find one with the stat in the center of the manifold.
If you are looking for the best, performance wise, try and score a 71 manifold without the EGR. There is a part number the SS guys look for, but I don't remember it. Seemed like it ended with 100 or something.
I have a non-EGR intake, but I cracked it screwing the bypass hose fitting into it. :-(