Welcome to Lemons and another hopeless Z car build. I am helping yet another delusional friend build a Lemons Z. I am not sure which addiction is worse, Lemons, Zs or Zs in Lemons. A few more years of this and they will have to make a special hospital for all us nut jobZ.
My car has less the 4" of ground clearance as it is. We have bottomed out the frame rails and been pretty hard on the exhaust.
Our short tires lower the car about an additional inch. If you drop the floor 3" I don't think you will clear much of anything. That will also affect the frame rails and overall strength and integrity of the car. In my opinion, this is a bad idea.
I replaced my floor pans and installed seat rails out of 1.5" angle iron. We run a Kirkey seat on modified OE sliders and I am thinking our butts are about 2.5" off the floor. I've had plenty of 6'2"ish drivers.
My friends car with a Corbeau seat puts the driver higher up which is not as easy on the tall guys.
Get a seat and see how it works in the car before you start dropping the floor.
I replaced the floor first. Then we stuck the initial seat in the car and played with pieces of wood to get the seat height and angle.
We upgraded to the Kirkey seats for the second race. At that point I tried to put rails as close to the floor as possible while keeping everything inside the car. While there is some height and tilt adjustability in the arrangement, we have it as low as it will go. We made a booster for short drivers.
Something else to consider is a Hans recommends a 20 degree layback. Without a Hans, it's typically a comfortable angle too.
Troy
#35 LRE
1973 Datsun 240Z