First try lifting up the bottom of the seed drum when you tighten up the front knob. Also, you may have remove a washer or two from behind the front plate so the drum can move back further against the firewall.    Chances are lifting the drum up on back will solve the problem.  Just sliding the drum on against the manifold and tightening allows the back to sag.   Been down that road

If that doesn't work, then you need to align the shaft that comes out of the seed hopper.   To do that, you need to remove the back panel behind the hopper and slide the gear box to correct.   In your case the  gear box would go up so your alignment on the front of the shaft goes down.   CIH says use a swing gauge but you can probably do it without.   I rebuilt my gear boxes a couple months ago and got it lined up without one.