[Aussie Mike]

I don't think it matters too much what size baseplate you use. The CA425 operates on vacume much like a CD stromberg. It has a diaphram that pulls a slide up allowing air to flow through. In the bottom of the slide is a cone shaped knob that meters how much gas is let through. A screw in the side of unit moves a plate back and forth to restrict the size of the opening and set your overall mixture. There's a seperate idle mixture circuit on the side which lets you set your idle mixture.

I believe there are a couple of different slides with different shaped end cones to give a couple of jetting options much like different needles for the jets in a SU or Stromberg carb.

A CA425 will only flow 425CFM wide open so the base plate probably doesn't matter too much. Smaller butterflys will give you a bit more throttle sensitivity. The mixer bolts directly to any of the Holley square bore base plates. I believe the 465 and 600 both have the same size butterflys under there.

I built one for my F100 years ago and just picked up an old Holley from a swap meet for $25. Take a vernier caliper with you to check the sizes of the butterflys if you are looking for a matching pair.

Most of the donor carbs that you will find at swap meets etc are Vac secondary but it's pretty straight forward to make them mechanical. A bit of welding on the linkage and an extra spring for the secondaries.

Cheers