Doweling the screw holes is a recommended fix. Even the period Fisher Body Manuals recommended it. Just make sure that you get a tight fit and use a good water proof glue.
I have never used the body filler or other products. Having spend a bunch of years in Urethane chemistry I know those can be great products so should work well. My major concern is making sure that you get a good bond. Grease, oil, wood deterioration, etc. can present problems.
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How Sweet the roar of a Chevy four!