The Auto and the 4 speed cars which were delivered with no OD came with the same 4.10 back axle so revs at, say, 70 mph will be approximately the same. OD cars had the 4.56 axle the same as the wagon, although the wagon never had the OD from the factory. Fitting the OD to a car with the 4.10 axle makes it a good cruiser, quieter and more economic if used properly. By that I mean clicking it in above about 40 mph and not allowing it to lug in OD.
In good tune the auto will have no trouble keeping up with the traffic. In Europe the auto cars only came with the single carb engine so you're one step up from those. Having said that the B18A and B20A with the A cam is very sweet and torquey at low revs and very relaxing to drive and matches the auto box well.
Go see the car, if it checks out as regards condition, then go for a drive. At $2500 it should be pretty decent with no serious rust. Beware if it's had recent paint and the under hood area is scruffy. Check it out underneath and pull the mats, inside and in the trunk to check for rust. If the seller doesn't want to cooperate you can guess why. It's always "buyer beware" don't forget! If you like it, buy it.