The "621" bellhousing, 11"clutch, and 14" flywheel setup you have is not correct for a Z/28.
The "403" bellhousing, 12.75" flywheel and 10.5" clutch setup is correct. You can use the "621" but the starter nose cone, flywheel and clutch will ALL be different.
Try a BB nose cone if you dont want to go find the correct "403" setup. You can not mix-and-match these setups - ALL of the parts listed will need to match one setup or the other setup. Been there, done that. Even the starter brace would be different. None of the parts you have will be technicaly correct except for the non-fitting starter.
If it was mine, I would sell the "621" for good money and find the cheaper and more plentiful "403" bellhousing, flywheel, clutch and use the starter you have so you don't have a "MUTT" car later on down the road.