Hy reidy, I'll give you the bad news first, the only automatic transmissions that will bolt up to the 1958 Chevrolet 261 WITHOUT AN ADAPTER are the Powerglide and the Dual Range Four Speed Hydramatic. All other transmissions be they three speed or four speed will require an adapter between the engine and transmission. I obviously have no idea what kind of facilities you have, or your ability to have an adapter fabricated for you in Australia to save the shipping costs, or if that option would ultimately end up being cheaper? I think you have some more investigating to do before you proceed, good luck!
