The bolts holding the inner fenders to kickboard on my truck were hex, and I'm pretty sure they were original. The LF fender was replaced but the right was original.
But either slotted heads or hex can be ground off like you suggest. Much easier to work on stuck bolts when you're not laying on your back underneath or trying to squeeze under the dash.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.