On my '51, all the bed fasteners except the fender bolts were hex head. And like you suspected, the bolts were installed from the outside. And they did have lock washers. I would assume that earlier trucks were the same. Front panel bolts were 1/4" except the rod at the top roll is 5/16". Length - I'm guessing 1/2" or 3/4" (just enough so the end of the bolt goes all the way thru the nut.) Rear cross sill bolts a mix of 1/4" and 5/16", you'll be able to tell by the hole sizes - use the biggest that will fit the holes.
Don't know about the rocker panels.

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.