Where to buy correct alternator for a 1969 340 swinger non a/c car

I need some advice on where to buy the correct alternator for my non a/c 340 swinger. The original alternator is long gone. I have a stock wiring harness and a new old stock 69 voltage regulator. It seems like most of the aftermarket and rebuilt alternators available are much higher amp rating than my stock one should be.
Oreilly shows both single and double pulley round back 55 amp available.