How much weight can threads hold?
The 1/2" bolts will hold up your truck, or thereabouts, so that's not an issue.

I'd be concerned about the pullout strength of the deck screws (especially if your rack was particularly big or you have a fondness for cast iron). Upgrade to something a little bigger. I'd lean toward a 1/4" lag screw, 3" or so, assuming that fits the flange. If that doesn't fit, aim for a #12 deck screw. Predrill pilot holes for the screws.

A little research... in typical conditions, you can expect a 2.5" fully threaded #12 screw to hold something like 300# in pine. See the calculator at engineering toolbox.