can't really say for sure, but 1/8'' and 1/8'' add up to 1/4'' of slop in your plan
i'd drill them the exact side of the washer and bolt, tap them in if needed
the washer is eccentrically drilled so you can use the square stove bolt and lock it in from turning
do a test hole in scrap oak or cherry to see if your plan is doable, as is, i think the head will turn
typically the bed rails are above the bed wood as a shovel guide (shudder
)
so as your shoveling gravel (ouch) or asphalt (gross) out of the bed the shovel doesn't dig into the bed wood
if you painted the strips and bolts, recess the strips and bolts a bit under the surface of the bed wood
this is a little deeper than the plans on the bed wood site
and if you sprayed the bed strips... did they take as much paint as half your truck or what???