The 6.2 and 6.5 is simply a bean counter engine finally fixed by AG to hold up a little better. It was made for MPG and to be cheap above and beyond anything else. No intercooler even when Dodge put one on. It did not have to last over the 100K warranty some came with, most had 3/36K.
Like this is how you become the low bidder for gov't contracts, eh?
Now the 6.5 is just a bored over 6.2. The pistons are thinner to make the rotating mass the same. The 6.2 heads are different and you are best to use 6.5 heads on a 6.2 for injector clearance on the turbo manifold. But you can jam it toghether with 6.2 heads pretty or not it will work. (Cept the course thread injector heads.) Thicker pistons take just a little longer to burn through...