I am ~6' 4.5" and have my drivers side seat riser out as well just waiting on making the decision where to weld it in.
I was leaning towards about 2.75" back based on rough tests I did with my seat bolted to the replacement riser (I don't have any pedals or steering finalized in the car yet, so I just bolted them in to see how it would be). I had to guess on how high the pedals will be off the floorpan though which is why I am unsure. I guessed about 8" high for the brake pedal, does anyone know for heights for power brake and clutch pedals? I already welded patches in for the original seat mounting holes, figured I will drill new ones at the final location.
In addition, don't really like how the front of the seat riser will not have the support of the front frame rail extension anymore if I relocate it backwards.
Seems like a lot of work, but I will likely install spintech subframes to help support the floorpan and seat riser now that I'm moving these back.
I never really thought about having to move the seat belt mounting points, I guess that one is now on the list as well...
I figured I would set the pans as far back as I need them (maybe even err on the side of too far) and could always pull the seat forward if needed since I sit all the way back on the seat track. I wanted to drop the pan as well, but it just seems like a LOT of fabrication work and I'd probably weaken the seat riser. I think I am going to leave it as is and use the MTF headliner and perhaps find seats that are lower.
I'd be interested to hear what you end up with and how you like it; but frankly 3/4" doesn't seem far enough back to me personally. I will let you know what I end with once it's done if you're interested..