This is the email I got from Peninsula when I asked about the P400
Michael The P-400 has an oil pan that is three inches lower than the standard 6.2 / 6.5 oil pan (in the front) of course. Peninsular has made a deep sump rear and shorten up or made the front section not as deepso it will fit a std GM truck. It is expensive of course.We also have an aluminum pan same as the military and that has a deeper section forward so that will not work.

The P-400 is 501 more than the performance 6.5 then you still need the special oil pan. So you are looking at 1100 more than the standard engine. The real problem is there are no Oversize pistons for the P-400.The 6.5 in the performance piston there are . 020 and .030 OS. Yes these engines are set up for a serpentine water pump, flat water pump pulley, plus all the other accessories.

After I got this I emailed AM General asking about the differences in the engines and parts interchangeability. This is their reply.
The P-400 is the heavy duty version of the 6.5L engine. It has a forged steel crank instead of the standard cast iron crank; the con rod, piston and oil spray is different; the block features a bed-plate instead of the conventional bearing caps; the oil pan is structural casting instead of stamping and the heads have valve seat inserts.
The after-market may have not put in place any oversized pistons, as the engine is only a year old and the volume demand doesn’t warrant oversized production.
I knew about the girdle and crank, I also already know you can order an Optimizer with forged steel crank for about $700-$900 extra so the cranks are interchangeable. It seems the biggest internal difference is the rods, pistons, and oil spray. What is the difference in the pistons? I noticed he says the engine is only a year old. I'm sure I've been reading articles on these engines that date back to 2010 and maybe as far back as 2008.