It's not just the angle, the rear panel looks different. They probably reinforced it for the flat bed since the Brute has a full box around the bed.looks similar to an AEV Brute conversion on a 60's but the cab conversion is slightly different.
this one looks like it has more sheet metal behind the rear doors. maybe just the angles.
If I was going to build something like that I'd use a truck frame that size and try to put a jk body on it. I'd think it would be stronger then chopping and extending it to get that length.I can't remember where I saw the build but it looks like the same as the one that they put a JK body on a truck frame. I want to say it was a dodge but I can't remember.