I have that on my 55 3600. The way I fix those is to "dimple" each hole so the hole edges are rolled inward. This creates more surface area for the filler to hold on to without building it up high. I fill with fiberglass paste...the type that has ground glass in it ..not fiber. I then sand that and then cover the whole area with bondo..fo the cosmetic repair.
Of course if your gifted at fabbing sheet metal and welding it in that would be great but since your asking this question I sense you don't want to do that....It's a bad place to open up a can of worms.
Best of luck,
Keith
