At the present time i have two 1" steel tubes going across from the outside rails to the trans tunnel to mount the seats (on rails) to but i need to position the top seatbelt mounts about 2" above the top rail behind the seats. What i would like to know is whether a 2mm riveted steel floor could hold the weight of the seat and driver/passenger. This way i could remove the 1" rails and only need a 1" raise on the top seat belt mounts (and save weight )

yep that will be fine.

Mine will be bolted direct to a 1.6mm steel floor.

I have 1.6 mm steel floor as well . I use big spacers and washers. Why not stich weld the floor instead of rivets?

If in doubt weld in some flat strip across the floor at the relevant points. I used 50 x 3mm then mounted the runners on those.
Cheers, Pewe

mine are directly floor mounted - hold the weight fine, but i used mega big washed on both sides

MK Indy chasis has 2 steel strips instead of box section to mount the seats on - 50x3mm IIRC. My seats passed SVA bolted directly thouhgh these with large washers - rivetted ally floor.

Mines bolted straight to the floor.

Mine were bolted straight to the floor when they were the homemade plywood seats. Since we bought the Triton seats we installed seat runners and 1" tube to mount the runners.

I suppose it's possible now that my upper seatbelt mounts are too low ... shhh

Ok great. Will cut them out at the w/e

I really don't like the idea of bolting seats directly to a thin panel, especially one which is riveted on.

If you cut them out just make sure they aren't structural. Without them the lateral forces are all being absorbed by the floor over quite a wide span (no doubt a proper Mech Eng will be along to confirm or deny that).
The flat strips will help counter the loss of rigidity.
Cheers, Pewe