Body panel screw holes: Match pattern, or patch and make new ones?

[threewordphrase]

Heya folks! I raised the roof on my 1993 Thomas Minotaur - The Struggle Bus.

The plan right now is to reuse the existing windows, sills, small panels like the ones that go between the windows, basically everything I can, and over time as I live in it and get a feel for what I want the interior layout to be I will remove windows, get new ones, cover as necessary. Basically, I'm trying to defer big decisions until I'm sure what I want.

The topic in my mind right now, and I've been searching the forums and googling to no avail on the best practice here: I have the window sill pieces that were screwed into the top of the existing body panel with #10 screws. I am adding a 16" high body panel, which will be screwed into the top of the existing panel, and the sills will be screwed into the top of the new piece... pretty straightforward splice. Now, do I want to match the screw holes in the body panel and sills in the new piece, using transfer screws or something similar, or do i want to patch the old holes (I have a welder and a bunch of Bondo, would Bondo be fine or would I weld those holes full?) and then start with fresh pilot holes? A third option would be to do new holes where the new panel overlaps the old one, ignoring the existing holes since they'll be covered and the joint will be full of butyl tape, and treating the holes in the sills as oversized clearance holes and piloting the new sheet through those holes.

Tons of ways to skin this cat, and I can't for the life of me find any good references!

I imagine that reusing old screw holes, with or without going up a screw size, would risk mangling the steel and making a loose screw, or slight misalignments could pull the sheet around in weird ways and make it lumpy/wrinkly. But on the other hand, if I'm overthinking things (as I often do), it will help me avoid a weird patchwork of Bondo or days of MIG filling old holes.

Thanks everyone!

[Mountain Gnome]

Can you slip a sheet of paper at least behind the old panel with the holes already in it? Mark the paper through the holes with a pencil.

If the new panel only covers the space and does not support weight from above, the holes you pre-drill in it (using the patters you made in the step above) can be slightly (very slightly) bigger than the screw shank. In fact, this is preferred to allow for thermal expansion/contraction.

If this new panel is more structural, then clamp it in place, and drill new holes strait through both new and old. Just keep approximate track of where the old holes are and keep the new holes as far from the old ones as possible. I don't see any reason to weld the old holes closed, unless I really don't understand the setup, or the bolt-holes are large and this is all weight-bearing structural stuff, and the holes are either plentiful, or situated at stress-points. If the latter is true, and these stress-points were there at the factory installation points, then you may be back to trying to use only the original holes...

[Mountain Gnome]

In fact, I think I would use rivets to bind the new panel to the old panel, marking the old holes on paper and using the pattern to drill new, slightly larger holes. They won't vibrate out. Maybe even put in a few more new holes. Remember to allow for thermal expansion. Using rivets assumes you will be sure your construction is complete, and you won't (anytime soon) need to go back in the walls to add wiring or something.

Keep the windows screwed in. Hateful throw rocks. Trees fall. Winds blow. Etc.