Cruse go ahead and try it if you like, Its not a huge deal. If you go the washer route then a much better route to go and is easy, is to make a plywood plate that is long enough to span two firewall bolts either across for adding down thrust or up and down for adding right thrust and drill the holes sloppy. You can try several of different thickness very easily. After you pick one just drip a little CA between the firewall and spacer plate.

The trouble with the washers is they will always start to dig into the firewall after a while and the bolts will always be getting loose and if you keep tightening them you gradually lose the right or down thrust. This is an old controlline trick from the days of wood engine mounts.

John