Most kits build the correct offset into the firewall. If you are scratch building and not from plans, you may want to keep the firewall at 0,0 offset. I would think a couple degrees down and maybe one right would be a good start point if you want to build it into the fire wall. You can use flat washers behind the motor mount. Just make sure to add them in pairs. IE two top screws to lower the engine. Now 2 top and bottom on the left side to dial in a little more right thrust. What you want to avoid is getting the motor mount setting on an uneven base. That will put some twist into the mount and may cause early failure. The washers alone, even used in pairs is going to put a bit of twist on the back lugs.

Don