RO,
Mount your motor mount to the engine and center it on the firewall ( or desiredd position) then mark your holes. You may need to mark the mount's top and bottom position and remove the motor to get to the holes.

Here is a home made hole center marking tool. the conical tip is from a ball point pen, the barrel is a peice of brass tubeing, the inner piece is a sharpened length of music wire, springs from dead pens with a servo grommet for the spring keeper and a trusty wheel collar to top it off works great.
Sorry about the pic size[]