Gday again,
Just want to clarify a couple of things:
I measured up one of my 15x10 Weld wheels (with 295/50/15) this morning, the wheel's back spacing (not including the tyre) is 5", but that is insufficient. I bought these wheels second hand, but if I were ordering new wheels I would order 5.5" back spacing.
The wheel/tyre will sit close to both the leaf spring and the guard/fender lip. To gain a little more clearance you can roll the guard lip, and/or, do the old Bathurst Falcon coupe modification of slightly bulging the guards (I haven't tried this on a ZH, but it works well on coupes).
The rear axle of leaf sprin cars tends to move from side to side quite a bit, so the likelihood of tyre contact with spring, or guard, is still high. You could try instaling a panhard bar or a watts link to limit this movement.
Be aware, also, that from the factory due to manufacturing tolerances,slight missalignments in manufacture etc. the rear axle/diff is often not dead centre in the body/frame (mine was off centre to the right by half, or three quarters of an inch, can't remember which). This is not a problem with stock skinny wheels, (and you cant even see the missalignment unless you measure everything) but with BIG wheels/tyres it becomes obvious.
Also, with such large tyres you may not be able to lower your car as much as you would like, without rubbing.
On top of all this, if you do your homework, and the required hard work and manage to get the lot to fit you will still find possible legality problems, some of which are:
-over track
-on a 9" rear axle the wheel rims should have a ZERO offset (at least in S.Aust. I think that is the case)
-depending on what wheels/tyres are fitted to the font of the car (i.e. not wide enough), the rear tyres may be deemed too wide.
SUMMARY: 15x10 rims, with 295/50/15 tyres can fit on the rear of a ZH, but you would have to really, really want them.