I have a 2 door datsun 120y sedan with a stock A12 engine. I have an A14 engine to put it on with a 5 speed transmition! What diff should i put it? Is there any close bolt on solution? Will an Vanete diff be a good solution?

Do you want to change the differential because the ratio is too low, or the whole rear end because you think it's too weak?

If it's the diff ratio, then a 3.7 to 1 & a 3.54 to 1 is available for the H150 which is what your car most likely has.
I know that these are available in the US, so if this is what you need then it might be an idea to talk to some of our US members.

If you need a lower ratio for the H150, then you will need to check the Pitroad or Nismo catalogues/websites as there was no ratio lower than 3.9 to 1 in a factory production model that I know of.

If you want a stronger rear end then you must plan on dishing out a lot of abuse as the H150 differential was the stock item behind all A15 passenger models including the heavy B310 wagon.

If you are running a mild A14/15 in a B210 model then the original H150 size differential should be just fine.

thank you once again D. I think i will stay with the stock diff