Metal on the bracket looks beefy. I’m not so sure I would be in a hurry to paint the inside black. I like how it looks now instead of the coal mine after dark effect. I personally would be inclined to match the grey that’s in there and see how it looked. You could spray some matching grey primer on the rusty spots as a test. The bed actually looks pretty good from here. Maybe a wash down and some poly and roll with it.
Just like in a house, dark colors tend to shrink the space to the eye, while light colors give a bigger feel. But hey, it’s your truck after all.😀