"Filling" as in welding up the unused holes? I'd be very reluctant to do much welding on a frame, as it can produce all sorts of unwanted stress points. You might glue a small piece of sheet metal on the inside of the frame behind the holes you plan to fill, and use Devcon epoxy steel to fill with. It's sort of an industrial-strength JB Weld, but much stronger. Once the patch is sanded and painted, it would look like the hole didn't exist.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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