All of the GM castings are suspect for the cracking issue. Some worse than others.
The later years are actually worse than the early years.
Casting numbers are you best bet to find a good one.
599 castings are reported to be the best, but I've had one that cracked in the cap registers, so no guarantees.
506 castings are generally agreed to be the worst, a lot having to do with the tooling errors when making the piston oil cooling jets in the main webs.
GM castings seem to crack everywhere: main cap bolts, Cap registers, oil cooling jets, cylinders, heads, starter mounts, etc. Someone even posted up once where it had cracked around the head bolt holes.
Now if you do find one that is not "euchred" by now, it's probably a fine candidate for a stock power level rebuild. Generally, if it was going to suffer cracking it will show signs of it by the time is makes it to a "worn out" rebuild vice a catastrophic failure death. No guarantees though, it's still a comparatively weak GM casting.
You best bet to get a block that will be in good shape, drop in like OEM and likely to not suffer the cracking issue is a GEP Optimizer block. These were cast by IH for GEP (AMG subsidiary) to supply re-power engines in the HMMWV for the re-power/up-armor project. these have "506" cast in the block, but they also have the IH symbol cast in the valley. You can get these from surplus suppliers if you ask the right questions and shop smart. Ted's trucks are/were a popular supplier, but it seems his stock has been drying up lately.
The optimizer has different metallurgy, is a high quality casting by IH and has changes in it production process (like shot peening parts, etc).
This is what I have in my truck and it's getting on for 4 years and running strong. It has been "massaged" to 300HP and 500 Lbft crank (estimated), 271 hp 446 lbft at the wheels (rear wheel dyno) with no issues so far. Granted, I haven't pulled the oil pan off for a look, but I've had "eyes" in there a couple times in the form of a boroscope. Same one we use to inspect compressor blades deep in to the compressor section in jet engines. No cracks to date. My engine also gets the best of bits, Fluidampr is one of those, but not a forged crank. My 6.5 is pretty much at the end of it's development life for me. If/when it dies, it's coming out in favor of a 6BT or a Duramax.
The final word so far on a crack resistant 6.5 is the optimizer P400 casting. Major redesign of the block while retaining the "drop in" servicing for the heavy up-armored HMMWV's, but requires some oil pan work to fit in a gmt400 4x4 chassis. But you'll be in to it for $5-8,000 at this point as they are currently not available as surplus.
One day, it will be. But not today.
