Hayes calipers are designed to fit the 74mm post-type mount. I like the post mount better, partly due to the fact it can be adapted to the 51mm IS mount. The other way wont work. You should be able to mount the caliper without adapters/extenders to make it work with a 6 inch rotor.
Aloha Endo, You will need the hayes IS to Hayes adapter. If you're going to use the 8" you'll need the same except for an 8" rotor. Your LBS should have one on hand. Especially the 6" version (much more common). Aloha