You can make 30-40 from .303, but it will be short in the neck.  I wasn't aware you were in Australia.  I've seen some nice Krags from there, one had an old long target scope on it that was worth more than the rifle.  With the new laws prohibiting export of almost anything gun related, you'll likely have to get someone there to turn you a barrel.  That won't be cheap as blanks that will finish out at 30" are expensive in themselves.