I dropped the floor on two different GTD's, mine and Rick Merz red beauty. I welded up a sub frame out of aluminum and bolted it to the existing frame rails. It is angled at the front so it will slide over things if you bottom out and runs from about were the center shift mount is aft to the beginning of the engine compartment. The exterior is clad in heavey gauge stainless steel with welded joints and sealed with sikaflex, just like the original floor pan. We cut out the bottom of the fiberglass tub and extended it for the drop. Make sure the old floor pan extends over the top of the leading edge of the drop section and is well sealed with silaflex and rivets to keep the water out. I also used thin sections of hard foam bonded to the floor pan and the tub to stop strange noises from vibration,drumming etc. It has worked very well and is easily done when building the car - a lot more work if the car is already finished. You can also gain a little head room by spacing the entire body package upwith 1/2" spacers. You will never see the difference if done right. My car is not spaced up but Ricks was and it looked great. Now take out the seat adjustment runners and bolt direct to the mounting plates and you have gained about 1 3/4" to 2" of head room. See attached 15 year old pictures - they are all I have.