The stock bracket has a bolt head that sticks down on that inside edge since it's a two-part strap, with the horizontal flat part from the skid plate meeting the vertical part coming down from the frame, then being bolted together, sandwiching the tank with the skid plate.
More bolts that don't hang down any lower won't present much of a clearance issue, but they will of course provide teeth to grab stuff that would otherwise slide along the smooth skid.
That wouldn't be my biggest concern, though. Part of the reason Ford used rivets for the straps was so that there wasn't anything that could puncture the fuel tank. It may be plastic but bolts are certainly capable of popping or wearing a hole in the bottom of the tank, even if only over a period of time from movement or vibration, or in the event of a collision or hitting something hard enough that pushed the skid plate into the tank.
I'd say use something like pan head bolts so the smooth head is on the top, then tack weld those to the bracket or make square bolt holes for pan heads with the rectangular part under the head so they don't rotate, then use nylock or metal retaining nuts on the bottom to hold the tank.
Really, the best thing would be to just weld the strap to the skid plate, so there's no fasteners at all other than the one in the stock location, putting the skid plate bracket and frame bracket together.
When I pulled the skid plate on mine to drop the tank, I noticed all the factory bolts holding the skid plate and the tank straps were way longer than necessary and it took way too much effort to remove them since that much thread sticking out made a magnet for heavy rust, and of course the exposed threads were hard to get to with anything but a small wire brush and some spray lube.
I replaced all of them with shorter class 8.8 bolts, theres still a few threads sticking out past the retaining nut so they're safe and won't loosen. I also used regular loose washers since I couldn't get flange heads in a metric size.
