Welcome to the Forum.
There’s no way I would use the stock bumper to carry 300# or so!
. Having 300# sitting static on the bumper and not moving is one thing but going down the interstate or highway at 60 mph and bouncing up and down a completely different animal!
I removed the ends of the bumper leaving it attached to the frame rails, then re-enforced the frame rails with 1/4” angle iron going forward 4’, This was to help the frame support the new rack and the dynamic forces that would be in play. I used 1/4” stock on EVERYTHING, which is significantly stronger than the 11ga steel on the rear bumper. I also added a receiver designed to flat tow my Jeep with behind the fifth wheel.