I believe the two groove water pump pulley uses the rear larger dia groove for the alternator, the smaller front groove is used for the smog pump, if you don't have or use a smog pump you only need a single groove WP pulley the same dia as the larger of the two, double check.

Currently, the rear pulley is the smaller one and runs the alternator.  The front pulley is the larger dia. and runs the PS pump.

Thanks for your reply.

I'am sorry, I thought we were talking about water pump pulleys, right now I'am looking at a 53-72 GM parts manual to see if it gives the pulley dia but no luck, it shows the pulley arrangments for all but 70, so I'am gussing its the same as 71-72, if it is you should have a 2 gr. on the WP, 2gr. on the vib. damp., a single on the PS, and if you run a smog pump you should have a 3gr.on the WP instead of a two gr.
     All this BS isn't helping with your problem with pulley dia's, are you changing to alum. or chrome pulley's or are your org. missing? I'll look around and see if I can com up with some dia's somewhere.

Went out this morning and measured the pulley's on both cars and they are different, I sugguest that you take a "outside caliper" and put it on each pulley then measure the opening with a tape, you will get the correct size without taking the pulley off the car.anips38637.5542824074

Yes, I'm talking about WP pulleys.  I'm asking because the previous owner threw together a pile of engine parts he probably had laying aroud the garage and I didn't spot all this chaos until I tore into the motor.  The assembly manual is specing out a 2-groove WP pulley with part number 3890419.  I've also seen it listed for a '67 Camaro.