Jester,
Please let us know what if any difference there is between the AA gears and the Atlas gears. The only thing that ever seems to get posted on this subject is that someone will write that he "thinks" that the gears are interchangeable.
The drill chuck will have to have the arbor changed as if it fits the AA, it is #0 MT. As Rich wrote, the 618 tailstock taper is #1 MT. There probably isn't any practical or economical way to use the 4-jaw chuck on the 618 if it fits the AA.
If the AA is a Dunlap without lead screw it is a 109.0702. If it has a lead screw (since it has change gears, it must), it is a 109.0703. In either case, the spindle nose threads are 1/2"-24.
Otherwise, it is either a 109.20630 or 109.21270 and has 1/2"-20 threads.
The Atlas 618 has 1"-10 nose threads. The only Atlas built metal lathe with 1"-8 threads is the 101.07301. The 101.07300 (only built for one year 1938), had 3/4"-16 threads.