When I tore down my front calipers for rebuilding I encountered the rust pitted pistons problem. A little searching about and I found that 1979 to 89 Toyota Hilux/FourRunner front calipers have the same staggered bore sizes as the Girlings. So I bought a set off Ebay and stripped them for the parts I needed.
For about $100 I got eight new pistons and rubber parts that fit perfectly.
Only caveat is that the Toyota pistons are just a little bit shorter than the Girlings, and if you elect to split your Girlings, you will need the pegasus seals.
Good luck.