I painted up my new arms (thanks Harrmac) and had them ready to install. I pulled off one side (I thought I'd better check them before I go putting it all back together) and they are identical to the arms I already have.
This means that I basically have the maximum amount of Castor available, and that's with the offset arms. If I had to use symmetrical arms I would only have about 1 - 1.5 degrees
I think after rego I'll need to do some front end "re-engineering"....