Lowering the drivers seat?
 

I'm 6"3 and with a helmet I just barely am able to drive comfortably at race events.

Does anyone know a way to modify the stock seat or seat rails to lower the seating position. Or does anyone know of an aftermarket seat that will do the trick?

i have a sparco sprint in my car with rails i made myself its pretty much on the floor

I think I'm going to custom fab lower brackets, might have to cut into the floor. It's simply a low car. I measured mine at about 49" tall at the peak of the roof (it's lowered only about an inch, though obviously that doesn't change the cabin size, just illustrating how low they are...).

I've heard the bride seat rails help with lowering the seat position but you have to get a side mounting seat. this is the route I'm going. I'm the same height as you if this helps

I am 6'3 as well. You are going to need to get another seat.

I have a Sparco Sprint bolted to the OEM slider with a couple pieces of steel. It works great and I fit in the car fine with my helmet.

If you keep the stock seat(s), your easiest path is to simply remove the sliders and fab some brackets that bolt the seat directly to the floor.
That should net you about an inch of extra room.

If you have a sunroof (chances are good you do), removing the sunroof frees up even more headroom (and removes some weight from up high as well).

Nismo, you don't mention whether this is a track only car- in which case your options are much more numerous- or a street car where aesthetics matter (specifically, will there be a passenger seat and do you care if the seats match?).

If there will be two matched seats, start with the passenger side because it's by far the most difficult to mount. Once you've conquered the hard side, the driver side will be cake.

ok so what should i do? im 6'4 and with the top up, my head touches the roof with no helmet. when the top is down, the top of my head is higher than the windshield

Something 'easy' is you can remove some of the seat cushion padding, I did that and it helped a little. I'm 6'1".