The Mustangs Plus catalog has (or has had) an article with pictures about lowering the seat platforms to gain more head room and leg room. The article states that their procedure can be adapted to actually move the seat pans back, but gives no guidance.
Mustang Monthly had an article 6-12 months ago on relocating seats in a late model. It involved some flat straps to extend the seat box back. I do not see why that could not be adapted for early model cars.
I had to put new seat risers in anyway, so I cut them down 13/4 inches and bent a new flange, then moved them back 2 inches before welding them in. When I trial fitted the seats, they are PERFECT! I'm 6'4" also.
I also moved mine back.. FYI there may be an added bonus.. I am 6'5" my girlfriend is 5'2"...the seat now wont go far enough forward for her to depress the clutch...oh darn she cant drive the car! LOL
I just finished remounting my seats for the same reason. All I did was use the rear seat pan holes for the forward seat bolts and drilled two new holes in the rear seat footwell for the rear seat bolts. I made a mounting braket to account for the height difference and to strengthen the floor pan. It worked out well but this winter I will probably also do the seat pan mods so that the seat also sits lower to gain more leg clearance to the steering wheel.
I took an approach which required no modification to the
floor. I separated each of the two seat tracks from the
seat and fabricated four identical 'bars' from flat steel stock. I drilled the necessary 2 holes in the end of the bars. The seat is now positioned rearward relative to the tracks. It's pretty heavy stock to handle the load. This
mod moves the seat back an additional 4 inches without
altering the seats, tracks or floor. The only drawback is
that the adjuster lever is 4 inches in front of the seat.
It's not really noticable and doesn't get in the way.
I did the Mustang Plus method and it worked out well, with one small problem. The seat now sits so low that it rubs against the transmision tunnel. I have 69 Mach-I seats in a 65 f/b and have some pretty thick sound insulation under the carpet. That might have something to do with it. Sure beats rubbing your head against the headliner!