Kirk is correct - the earlier SOHC 400's had a flat stamped top clamp, will not work for you.
350 clamps will not work at all on a 360.
The 360 clamps are notorious for cracking - there's a specific tightening procedure you MUST follow when buttoning up the front end.
My DOHC 450 mutant uses 360 top and bottom clamps (with a welded in 450 stem), and I made sure to have a spare clamp on hand - they're getting hard to find.......