Determine the thrust-line of the design and mark both the thrust-line and centerline on the firewall (thrust-line = up and down location and centerline = port and starboard location. This should end up looking like the cross hairs in a rifle scope. Make a hole pattern template of some stiff material that matches the existing hole pattern on the firewall (I like aluminum as it will keep the drill from wondering during drilling). If needed, drill out the existing holes so dowels can be inserted to fill them in and sand flush. Rotate the pattern template the way you want, align with cross hairs on the firewall and drill new holes.

You will need to raise the fuel tank to accommodate the new-higher carb. level.

I agree with your decision to do this! Why manufacturers have decided to design airplanes with inverted engine installations is beyond my comprehension!! Yes, they seem to work better for 4 strokes, however, I still think it makes life more complicated!