ed,
You want the pulleys on your harmonic balancer, alternator/generator, and water pump to match.
So, my guess is that you will want to match to your 1960 harmonic balancer (3/8" belt/pulley) with a 53-55 water pump with relocator plate (go for a 1954 water pump/pulley and avoid confusion).
I'd stay away from a 53-55 235 Corvette water pump because the Master Parts Book shows it as being different (maybe because a special generator/tachometer-drive was used on that Corvette?).
Tim
