I agree, the loop cluster would be the way to go, keeping it original. Using the 66 cluster, you are running wires that are not supposed to be there for the 65, and omitting wires that are supposed to be there for 65 in order to get that 66 dash working.
Another question that may be raised is... if it is a Luxury interior car, and the 5 gauge cluster was supposed to be there anyways, wouldn't the wiring be there as well? Basically it would be just a matter of pulling out everything that has been done to add the 66 cluster, and reconnecting it properly.