Something I tried recently was to position as close as possible on the firewall and drill the holes. Then make a plywood ring to match the back of the engine mount including the mounting holes. Put the blind nuts in the plywood ring. Enlarge the holes in the firewall to allow the bolt some movement. Now bolt the mount through the firewall and into the plywood ring with the blind nuts. You can then loosen the mounting bolts and move the mount within the confines of the enlarged firewall holes.