For my two bob, I would very strongly suggest that you get the rear axle assemblly that was designed for 1200's in the first place, the 1200 Ute item It may take a little bit of searching, but it will save you a lot of work, grief, & disappointment in the end. It will probably save you money in the long run too.

The Vanette diff will be a fairly low ratio & if you are using the car for street duty, & it is fitted with a good 1200, or better, engine, then you would be looking for another differential to fit in there anyway to stop it reving its little heart out in top.

If you fit a rear axle assembly that is too wide, you will need to use wheels with plenty of offset, which definately do not have the good visual appeal of wheels that heve a nice little bit of 'dish'.
Alternately, you need to find & fit flares & maybe even modify the wheel opening in the body in a worse case scenario.

Anyway, that's my suggestion, spend the time & effort, & money if need be, to find a Ute rear end & you won't be disappointed.

P.S. All 1200's have the same width rear end.