The last time I worked with a harness from Lightspeed the Dsub connector was of the solder cup variety and would have required unsoldering all the connections to remove the connector.  If this is still the case unless you, or someone you know, is comfortable unsoldering and resoldering the wires I would suggest using one of the techniques other LML readers have suggested. Removing the connector backshell then folding the wires over (carefully, so you don't stress the solder joints) so that the connector can pass through the hole with the long axis of the connector body perpendicular to the firewall should allow a hole as small as 7/8" diameter.

Tom Gourley