That is a real nice setup... and the thing I like the most about it is he went the extra step for an angled piece to bring the pipe up the center of the bed.
I don't know who it is, but there's a truck on DTR that has, by far, the best setup I've seen. Can't recall the color, but I think he has a headache rack and he also notched out his inbed toolbox to hide the pipe under it.
Oh yea... and I'd go Mitre Cut and stay away from the flappers. To me, they just don't look right on a pickup. I've already got the next truck planned out in my head and it involves stacks, I'm just stuck on having them exit straight back or angled out.