The 621 is fine. They were used on 350's as well as BB's so that should not be an issue. The fork needs to be right but so does the bellcrank.
I suspect your disk is in backwards. The disk spring hub is rubbing on the heads of the flywheel bolts and the extra "thickness" causes the pressure plate fingers to be already partially depressed due to the disk interfearance. Rip it all back out and verify the disk is installed properly. If this is the case, it would explain the adjustment rod being all the way out. I know that is not good news but if this is the case, it is better to know now.
William - the Z/28 used the smaller "403" setup and all the 327's did too. The 350's are a mixed bag in general with some using the "621" bellhousing and some using the "403" smaller bellhousing setup.