HELP! 66 cuda idles fine, but not when in gear

Since your under a time crunch, get it to idle correct.
My suggestion is plug the vacuum advance hose (golf tee, just like when you set the timing).
Set the timing for 18* or 20* and get it to idle as slow as it will tolerate.
Once its warm,
Adjust the mix screws. Turn in until it starts to die, turn each back 1/4 turn. (CCW so its richer than when it started to die).
See if it will idle in gear.
If so, leave it that way and try driving it.
If its OK...
Put fresh plugs and tell them if they floor or otherwise pound on it, they owe an engine. You have a contract, right?
This way if there is too much timing for higher rpm, WOT, you'll know by seeing shiny specs of aluminum on the plugs when you get it back.
With no vacuum added advance, there should be no problems with part throttle timing. Fuel milage and efficiency will be less than ideal, but it will run.