65 Barracuda - Idle and no start issues

Another simple thing, could be a clogged fuel filter. The mech fuel pump only working at idle for a prolonged period of time may not have the suction to overcome a bad filter, where as when the motor is up in the revs, the pump can overcome the blockage. Also, be weary of what kind of fuel pump they bought. Those autozone fuel pumps are usually junk.