I use a 13mm "flex socket" with two long extensions. The extensions have to be long enough so that the ratchet is turned from almost the cross member.
A regular socket with an flex extension will not work because there is not enough clearence.
Use two long extensions rather than multiple short ones. Using multiple short extensions has too much "flop" making it more difficult to get the socket on the bolt. Also the extension will tend to twist too much under the torsion.
Also loosen the motor mounts slightly will let the motor tilt more. Be sure to support the motor so that the entire weight of the motor does not hang from the motor mounts.