It all depends what year your ute was made in, whether it has a H145 or H165 diff centre. The easiest way I know of is the H145's ususlly have studs on the diff housing, holding the centre in, while the H165's have bolts doing this job. If you've got the H145, it'll take a fair amount of punishment, except if you weld the spider gears up, when it'll break both axles at the spline ends the first time you seriously drop the clutch. The H165 will take anything an A series engine can dish out, because the more horsepower it makes, the less immediate torque it'll have. I've had H165's behind 1600cc A15's & they weren't a problem.
If you want to go any bigger, the main problem is that things have to be cut down to suit, like axles & housings & such, which gets expensive, particularly if it's not necessary. Alternatives could be an R31 borg warner diff or maybe a Ford 8 inch diff, or the diff from a 520/620 ute. Hope this helps you find out what you've got & if it's good enough for what you want.

Thanks alot for the piece of advice given b310gx. My 1200 is 81 model, ill have a look at the diff tomorrow and post up. I'm not looking for a diff to handle crazy hp as it is only a daily and not needed but would like to upgrade to a better diff..

I welded the spider gears on my H150 which I got out of my B210. My A15 toasted it. I picked up an H165 and welded the spider gears again. My A15 toasted that one as well. I now have a welded H190 out of a 320 pickup. The only mod. I did was move the leaf spring mounts out about 2 inches. This one wont break even with the Z motor I currently have in it.