i heard on the grape vine that u can get a bluebird diff and just unbolt the 1200 and bolt the blue bird straight in .....apparetnly this will give better ratios especially when cruising on a highway at 100 it will bring the revs down ....whats the deal on cant find any info on the forums about a bluebird diff bolting straight on ???
whats the feeling around the place??

The answer to your question is NO,... & YES.
The Datsun 1200 car uses the H145 diferential.

The "H" is Nissanspeak for "diferential", while the number is the diameter of the ring gear in millimeters. As far as i know, only Datsun 1000 & Datsun 1200 cars used the H145 diff & the ratio in your car is 3.9 to 1, which is as tall as it gets.

However,......
The Datsun 1200 utility used a rear axle assembly that was made for the H165 diferential. This rear axle assembly can be used in Datsun 1200 cars. The standard ratio in these ute diffs is 4.11 to 1 which is good for hill climbing & acceleration, but not so good for cruising.

The Nissan Bluebird [model 910] that was sold in Australia, in the series 1 & 2 versions, mostly used the H165 diferential in 3.9 ratio, which matches the car ratio. There are two exceptions.

The poverty pack sedan with the four speed manual trans has a H165 diff with 3.7 to 1 ratio
The poverty pack station wagon with four speed manual trans used the larger H190 diferential.
I believe that the series 3 Bluebird uses the H190 dif asross the board,.. but i don't know for sure.

The H165 diff is also used in other Nissan /Datsun models, so with any luck, others will let you know what they are.

So now you're an expert.

This is an international club, so perhaps you could add some details to your profile so that at least we know what country you're in.
Now would be a good time to do it.