Yeah, gluing the doubler is a critical step. I was hoping you had gotten it right. It's about the only thing that could shift the firewall angle. That or building a banana and I took your word on that. Now that it's glued up it's plan B time. First fly it and see how bad it is. You could get a new mount (an ARF "clamp" mount) and cock the engine a bit sideways in it. Washers work but tend to sink into the wood, use strips metal instead to spread the pressure out. LAST choice would be sanding because IMO it is hard to do straight. If you had a 2 piece mount it would be easier, again IMO.

Just a few of the options. More will be offered.