There seem to be two types of metro caliper, one with a spacer and an earlier one without (See
this link).
I would be very caution about splitting a caliper and discarding the spacer (if that's the type you have) as the join needs to be hydraulically secure, i.e. no leaks.
A better idea would be to fit the vented disks as they're more fade resistant and (figuratively) cooler. If you have the calipers without the spacer, this is what you'll have to do.
You'll also need a Goodridge metro brake kit from Minispares or similar, or a kit developed in Wellington which you can see in the photo below. The Goodridge kit uses a small line to link both sets of pistons; the kit I bought has a small block which does the same thing without being overly complex.
You will also need a special set of bolts to bolt the caliper to the hubs, from Minispares etc. The shank is metric but the thread is imperial. They're not expensive and minisport.co.uk will have them on your doorstep in days.
I'm not sure if you will need Metro drive flanges. I have and you need to grind off the metro wheel locating lugs.
I just fitted my disks this afternoon. The supplier only had Brembo ones, which were about NZ$120 each.