Good deal. BTW, grade 5 is all that was ever required for SCCA racing seat hardware. Grade 5 has shear strength 80% of a grade 8, and the real issue is the tearout of the floor. Sounds like you have that covered, OP. Once you get some tubing under there, the bolt will be in double-shear, the force on the floor will not be as much in tear-out mode, and it will be all better all around.
I elevated some seats in a trucks once by 4".. .used 2"x4" rectangular tubing under each rail, with 3/16" wall; huge washers under the floor. They were not going any where without part of the floor. LOL
