I've got a similar problem with a Bride fixed back I just installed in my car.
Mine sits on custom-made mounts which are standard ae86 rails with sidemounts welded on top. They look to have about the same dimensions as the FG rails height-wise. I don't know what most people consider too high or too low, but I find this rail about 2-3 inches too high. There is very little clearance between the bottom of the seat and the sliders (only about 10-15mm).
I think my only option to get it any lower would be making up a custom mount with no slidability, maybe with 4 seperate pieces (one each corner) or a rail at the front and back?
I'd be interested to see what others have done to overcome the height problem that many people seem to run into with fixed back seats. Post up some pics of your set-up if you can, I'd like to see what some people call low, just to get an idea.
In the next few weeks I'll be having a go at making a REAL super-low mount, so any input will be appreciated!