stake pocket dimensions
 

I want to use my stake pockets to hold posts to support a tarp a few feet over the bed on my '72 fleetside, to provide shade while I sit back there in a lawn chair while parked at car shows and other events.

Best I can tell the stake pockets are not really sized to precisely accept any common lumber dimension. Is that correct, or is there something I should know before I start trimming down a 2x3?