Went through all the same problems with my GT40 seats including buying seats and they didn't fit.
I ended up with the Corbeau A4's, they tilt so their heavier but oh well. The first seats I had didn't have enough angle and I think most rigid seats don't lean back enough to give head room in a 40. Although there's one from Australia that I had marked the post but with the old forum so it's gone. They were $500 ea, the A4's are only $300, but they are narrow, my chassis is narrower than most.
4" thick on the padding is way too much. My bud's MK-V has about 3/4". He has some of that wonder foam in the bottom, and the seat has no bottom, the upolstered pad just sits right on the floor. The A4's have about 1-1/2" to 2" which I will need to thin for taller people, (I'm 5'6")
What I did to gain some room was to modify the sliders so the back of the seat was touching the floor and the front of the seat is about 2" off the floor. My floor pan has no ribs and it's at the bottom, lowest point on the car. The front of the seat can be up a little, it's the back that has to be on the floor, maybe you can remove one or two floor ribs and put strength back in with a thin flat plate.
My floor pan is .063" alum with an added .090" alum plate where the seat mounts. No ribs but one cross brace at the front of the seat. So the seat bottom is about 1/8" from the lowest point of the chassis.
Don't know if you should worry about matching the firewall angle what is more important is the seat back laying down for enough head room. When the A4 is angled for the most headroom it is a wider angle than the firewall, of course that means the bottom of the seat is a little further foward but it's a compromise either way.
I ended up with the Corbeau A4's, they tilt so their heavier but oh well. The first seats I had didn't have enough angle and I think most rigid seats don't lean back enough to give head room in a 40. Although there's one from Australia that I had marked the post but with the old forum so it's gone. They were $500 ea, the A4's are only $300, but they are narrow, my chassis is narrower than most.
4" thick on the padding is way too much. My bud's MK-V has about 3/4". He has some of that wonder foam in the bottom, and the seat has no bottom, the upolstered pad just sits right on the floor. The A4's have about 1-1/2" to 2" which I will need to thin for taller people, (I'm 5'6")
What I did to gain some room was to modify the sliders so the back of the seat was touching the floor and the front of the seat is about 2" off the floor. My floor pan has no ribs and it's at the bottom, lowest point on the car. The front of the seat can be up a little, it's the back that has to be on the floor, maybe you can remove one or two floor ribs and put strength back in with a thin flat plate.
My floor pan is .063" alum with an added .090" alum plate where the seat mounts. No ribs but one cross brace at the front of the seat. So the seat bottom is about 1/8" from the lowest point of the chassis.
Don't know if you should worry about matching the firewall angle what is more important is the seat back laying down for enough head room. When the A4 is angled for the most headroom it is a wider angle than the firewall, of course that means the bottom of the seat is a little further foward but it's a compromise either way.