Make the floor plates...
out of 3" square by 1/4" thickness minimum, and drill/tap the mount hole in the middle 3/8-NF thread. Only ONE threaded hole in the plates. When you know where you want the plates drill your mount hole thru the floor 13/32" and around the hole drill 4 smaller holes at least 5/16" dia. Mount your seat and snug up the bolts into your plates under the floorpan, then rosette or plug weld thru the smaller holes in the floor to anchor the threaded plate directly centered under each mount bolt. Nice and neat.