I had to use shop air to lift the gate open in order to get the inlet flange clamp installed because of the valve seat. The valve seat should be a fairly snug fit against your WG valve...
I'm assuming the WG location is a PITA on the 6.4 from the pics I've seen :shrug:
These pics may put the discussion into reference....I'm thinking you'd remembered this part if you'd installed it. It goes without saying...but without the valve seat being installed, you can see the potential for a drive pressure leak, and WG diaphragm heat damage, as exhaust gasses slip past the WG valve...
WG flange on a 7.3 dinosaur-
WG flange on a 7.3 dinosaur with the "valve seat" installed-
PTE 46 WG with "valve seat" installed-
Hope this helps. Don't know a thing about the 6.4 other than I wish I had one.