While the face full of ATF would be pretty darn funny, I would not feel good about drilling a hole in the pan while it is still on the truck. There is not a whole lot of space there. It doesn't make a huge mess when you take the pan off, you just need to be ready to catch the fluid. Once the pan is off the truck, I would think the bottom rear would be the best place for the plug. Just my thoughts...
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2008 6.7L Man Van
3500 4x4 68RFE
Mostly Stock, but LSD compatible