Just for these interested, here are some of the measurements i have come out with. Probably not exact cause the old seats will have been squished flat a bit. The bottom lines are the rails for each of the seats. I have done a mock fit up, and the new ones do fit, even just sitting on top of the old mounts on floor pans of the z. I should have about an inch to spare, font and side for the BMW rails to fit.

I've updated an older thread on installing earlier BMW seats and added a link to photos of the new seat frame that eliminates the BMW seat rails and uses the stock Z rails.

The thread link is here:

http://www.classiczcars.com/forums/showthread.php?t=35088

Dennis