[dsraven]

if welding a strip on the backside, then filling the gap from the front side, do you mean with another strip that fit the hole? like you would end up with a double layer area? I would pass on that idea personally as it becomes a stiffener and may affect the flexibility of the panel and it's ability to regain it's shape if someone were to use the step or whatever.
thats just me, your truck so you gotta do it your way.
there are also heat absorbing pastes you can get at the local welding supply shop. I have used them in the firefighter field for doing required rescue hot work next to a victim on vehicular extrication calls etc