I'm thinking about fitting a 3 point harness similar to the one pictured here
http://www.motor-world.co.uk/images/hires/6683.jpg

I want to fit the rear mounting point to the roll bar cross member, but it would seem too close to the back of the drivers neck. There looks to be at least a foot or two of webbing in the 'tail' of the belt, the roll bar attachment would only be a few inches away.

Would it be ok to have a mounting down low, somewhere near the diff and have the belt come up and over the roll bar cross-member?

Where else can I secure it?

Firstly, you can't mount it low as the rear harness mount point(s) have to be a certain height above the seat for SVA.

If you have a look around you will see that the harness companies make special shorter harnesses specifically for 7 type cars

HTH
Bill

even better are ther harnesses that have the lap strap webbing threaded the other way round if you know what i mean..........

instead of pushing the tail of the webbing away from you to tighten the lap strap you pull it towards you.

makes it a hell of a lot easier to get it done up right in the relatively cramped cockpit like a seven