The crazy thing is, the original track used the same bolts, and about half of them had NO nuts or washers on them. None had popped through or were even loose, but the rail is 1/4" thick and sometimes more at the crown. I put 3/4" washers, bent into a curve, on each one, except those too near bulkheads or hull sections. The track is eight feet long, and the way my sails are cut I will be not using the end positions, so it should be ok.
For deck mounts, that s/s backer plate sounds like a good fix.