Probably has to do with the gen 4 being dual disk if I remember correctly, so would make sense that there is more room built into the bellhousing for it and the slave would sit a little bit further back. The Viper style slave cylinders are known to have a pretty long stroke though, so probably not a big deal. I have a T56 in my 67 Dart and have run it with and without a 3/8" or so spacer and it worked fine either way. They are self adjusting within the range of travel they have, but never hurts to try to optimize it to have as much as possible available to account for clutch wear over time.
And the stock clutch in the Gen 3 is just fine, not sure what others might be getting at. I ran mine for ~100k miles if I remember right and put a stock spec Luk back in. So far it seems to hold the extra power my cam puts down just fine, though I do with it didn't have such a long pedal throw. That's my only real gripe with it.