I have used both clutches and see no difference in the function of the two on the GT500 and the '68 GT350.
Perhaps the C7ZX was created to give more room to the fan against the radiator itself on the '67 GT500?
The location of the fan hub on the water pump on the 428 is farther forward then the small block is and might have been created and used for that reason, i.e., more fan clearance to the radiator?
I also have one on my 68 GT350. I don't know why there was a need for it there other then an initial perception that what was good on the 67 GT500 would also be good with the 68 GT350?
Maybe it is just that the fan on the 68 with the A/C is a larger assembly? Don't know what the rationale was for the decision but frankly as Bob says, the value now is in being original on a difficult part to find.
As far as I can see, virtually any of the fan clutches will work fine. There seems to be no variation in cooling ability with the various hubs?
As a matter of fact, not to cast dispersion on Brown's shop at all, I SUSPECT that all of the hubs they sold and had remade were the same except for the stamping id on the flange? I may be wrong, but no one that I know of ever tested the repro's for variation.