when I weld something like this I do the edges but would also drill a couple extra holes towards center and weld in like spot welding to give better support , plus I plan on adding support on the bottom as well
The big deal with reinforcements is what happens in a wreck. The last thing you want is for the anchors for the seat to rip loose through the floor pan, thus my concern about reinforcements on top of the floor pan and held in place by 4 little welds. 72dustervs has a great plan!