The 599 block doesn't have the passages for oil cooled pistons, and therefore is a bit stronger than the 97 & 98 blocks. For later 6.5s, the oil passages were made smaller, and along with the thicker main webs these blocks gave the best of both worlds - strength and oil cooled pistons.
However, expect to pay a hefty premium for a new AMG 506 block. Based on my engine building experience, there is plenty of oil splash in the pan to properly lubricate and cool the piston and wrist pins.
That said, go where the value is. Look for a good deal on a later 506 block, but don't shy away from a solid non oil-spray 92 to early-96 block. They're not weak at all. /forums/images/graemlins/waytogo.gif
I'm using a CUCV 6.2L high nickel block, and it has worked out just fine under 15 psi of turbo power, and it didn't cost an arm and a leg to build.
As for a good place to find a complete engine - long or short block - the Diesel Depot has served many DP members well. Their customer service seems well above par; they offer several power level options, and they have a competitive warranty. /forums/images/graemlins/thumb.gif
Now, if you plan to build your own engine, then that's a whole other story...
