Craig,
Pity its not in NZ as we would be having a serious chat.
Could you tell me some more about the Corolla back end- is it ex a KE20 or 30?
Was it a straight bolt in and how do they compare lengthwise with the Datsun 1200 diff?
As I understand it the standard ratio for these is 4.3/1 which is a damn good all round sport ratio that would still give a good top end without reving the hell out of the motor and trying to find this ratio for the H145 is going to be an uphill battle.
Cheers
Dave