take off about 4 mm off your measurement in of the gap on the box. I would say buy the runners first to make sure but they would be roughly 1mm thick from the wood and 4 - 6mm in height altogether so if you take 4 off you'll have some room to play with
Just been thinking ( dangerous, i know!) If i measure the height on both vivs and subtract the difference and add it onto the height of the glass on the old viv, i SHOULD have the correct height measurement?
The glass runners are 2 diff sizes 4mm deep and 8mm deep ,well the stuff I've used is , so u put the 8mm runner at the top and 4 mm at the bottom ,you measure from inside the top runner to the top of the 4 mm at the bottom ,so the glass can be pushed up into the top and can just make it clear of the bottom runners ,then when it's seated at the bottom it's still in the runners at the top by at least 2 or 3 mm