I have just put a four speed in my 1000, now because it was a factory 3 on the tree I think the diff ratios must be pretty short as in 3.9's or something casue the four speed gave me no love when it comes to easy highway cruizing, I am still reving at the same RPM as with the 3 speed. My question is, what diff can I put in my 1000. I dont care about making new mounts or mucking around changing handbrake arrangements as long as the track is the same width. I can get a 120y 4 speed wagon diff, will this fit, and what will the ratios be?? Any advice on what will be a nice easy change over and help drop RPMs would be greatly appreciated.

The three speed box & the four speed box have the same top gear ratio, which is 1 to 1, so it will be the same to drive when in top. Some of the five speeds are 1 to 1 in top too.

The diff ratio in 1000's is lower than the 1200, which is 3.9 to 1, so to gear it up, use a 1200 diff pumpkin which will fit right in. The original 1200's tyre size was a little bigger too, so if you are using original wheels, then switch to 1200 wheels cos the're wider & use the 1200 tyre size, or else use mags & the right size tyres to match the 1200 ones.
Don't bother with the 120Y rear as it is too wide & so is the 1200 rear end & the 120Y pumpkin is a slightly bigger size & wont work as a bolt in. It has the same 3.9 to 1 ratio as the 1200 pumpkin anyway, so use one of those.

SO your saying the 1200=3.9:1 and the 1000= higher than 3.9:1. So is there anything that will fit that is around 3.7 even 3.45:1. What about 1200 autos, or utes. I can swap the centre thats not a problem just need to know what to get it out of???

Thanks for the help so far any one else help me out, i need to cruise at 60-65 MPH without doing 3.5k RPM+

If you read the relevant posts on this site you will find that the 1200 ute uses an H165 dif [which will not fit your H145 housing]
The 1000 car models also used 4.11 to 1 ratio's in their H145 diff which is a lower ratio than 3.9.

The 120Y & Sunny used a H150 diff of 3.9 ratio which will fit in your H145 axle housing with another gasket and a spacer.
The diferential from a very late sunny wagon that came with the a15 engine is 3.7 ratio and the 3.54 ratio was not used here, so you need to get one from overseas.
Bigger diameter rear tyres will help.