Mac Pro processor upgrade, won't boot
Processors either work or they don't. As long as they are siting in their sockets correctly. Tightening the heatsinks is unlikely to do anything, but be careful not to over tighten as you could damage something if you do.

You could take them out and verify the sockets are clean and free of obstructions and that there are no bent pins. You could also swap which socket which proc is in. I know that doesn't make sense but sometimes it just works that way.

Try with the old Procs in place if that is possible. If they work then you may have the wrong upgrade processor(s) or a defective one or two.