I have a 1975 Datsun 120y Model B210. I changed the original A12 engine with an A15. The engine works great, but I've already damaged the original 4 forward gearbox. I tried to get the 5 forward, but its really hard to get in my country (Trinidad). So, I ended up buying back a 4 forward, but when I shift in fourth the engine calls for another gear. Well my other problem is that my diff is completely damaged, It has a serious "Backlash". What I would like to find out is which diff could work on this model car. I'm not interested in changing the entire rear end, just the Diff. Could anyone who has done this before or has an idea as to how to solve this problem please respond, any help will be greatly appreciated.