How much different are they, really ?

The 67+ headers have much more room to work with. The only ones that work worth a hoot on a 66- are Cliffords. There's a reason they made them shorties, so you can finagle just two pipes around everything that's in the way.

I suppose if you really wanted to you could cut some later headers to fit, but I'm guessing it would be much more trouble than it's worth.