The factory box was no where close enough to being water tight, new it could drain through the holes all over where the metal strips meet the front of the box or out the back easy if the tilt was that way
But as dirt gets into the tiny holes yes it can hold water much longer
My solution was accidental
When I repaired (not restored) my box I welded metal strip parts from a donor truck and then made new square holes as needed for the bed bolts
The old original bed bolt holes are random and act as a drain

But maybe just park it pointing up grade and call it a day?
Else since the wood isn’t original anyway
Drill some holes? I think the solutions are as varied as the opinions that will follow
-s