Finishing an edge after a cut

[texastech_diesel]

A friend of mine wants a headache rack like I have on my truck, and making the rack isn't proving to be a huge problem. The issue is the bed rails are tapered, and he wants full length rail caps down his 8 foot bed; my rails are cut just long enough for a toolbox to sit on.

My truck:

Now imagine a tapering cut on the outside edge to match the taper of the bed rail caps.

I figure (roughly by eyeballing it) 1.5x4" angle iron for the rail caps, then mark the taper using a marker, and cut it with OA (that's all we have). After we cut it, how do we finish the cut edge so that it doesn't look like it was chopped down? I've tried using a small angle grinder to finish edges before, and they never come out rounded evenly down the length of the cut, they always turn out trapezoidal with sharp edges. And this is 16' total of edging that I'd like to make as uniform as possible.

Is there a die grinder bit that makes pretty, mill-like edges? Or do I need to practice with an angle grinder a whole lot? Any recommendations about rounding off long edges?