Check for a diamond marking in the valley and heads underneath the valve cover. Who knows...
I got lucky. So did
this guy. If you don't want to read the whole thread, in short, I came across a 6.5 for sale sitting inside an ice resurfacer. Wasn't even looking for one at the time, but it turned out to be a Navistar block.
That the engine came out of a 1996 truck does not mean it's a 1996 engine. Could've been replaced at some point, or even just the heads. Chances are small, but if you did get lucky that'd be good parts for a turbo build if in good condition...
Here's more info on how to identify the block.