BL:

Herewith a suggestion that may help in determining if the stumble is timing caused or carburation caused.
Re-assemble the distributor, assuring that the centrifugal advance mechanism is working freely, and that the point gap is correct.
Install the distributor, leaving both vacuum hoses disconnected.
Set the timing to yield 33-35* BTDC @ 3500RPM.
Drive the car without vacuum to the distributor.
If the stumble persists look to carburation, or, as a rather lower probably, a very weak coil and/or ignition amplifier (switchgear).