Matt, I looked into this quite a bit for my truck and decided against it. FETorino touches on a great point about the aero involved. The biggest thing for me was losing the use of the bed almost entirely. The next was the added need of a spoiler now that the tailgate was creating less down-force.
The hpt syclone also utilizes a similar design. A sheet metal wave helps to eliminate the cushion effect as air comes over the cab and guides it into the lower pressure zone behind the rad. The tailgate is then louvered to duct the air away.
http://www.highperformancetrucks.com...g2_itemId=9288
http://www.highperformancetrucks.com...2_itemId=11254
If you set the engine back, lighten up the front end and reposition the fuel tank you'll be in a position where moving the rad isn't going to be that much more of a benefit imo.