My bed, sitting on all new blocks and hardware, leveled and squared measures the following. In the center of the cab to my bed panel 1-1/8" (narrowest point with curve of cab). On the aprons to cab the narrowest point near the top of the apron is 5/8" clearance. If your bed is square and fenders correctly positioned it should be near these measurements. The clearance in your photo is too tight the normal flex is going to cause contact at those surfaces and mar your paint. I should add my truck is a frame-off so all mounts and related hardware is brand new, hope this helps.

1950 Chevrolet 3/4-Ton 3600
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