I take it you're installing aftermarket seats?
When I installed the racing buckets in my Fiero, I first tried using the stock rails with adaptor plates. The plates were only 1/8" thick (aluminum), but the seats ended up over an inch higher than stock. The seat backs are taller than stock, also. The end result? The tops of the seats were grazing the headliner. So I ended up using the rails that came with the seats (which have a lower profile) and making brackets to mount the rails to the floor. I was able to drop the seats about an inch.
Of course, your mileage may vary.
[This message has been edited by Blacktree (edited 11-25-2004).]