sitting for ??? with no plugs in.
 

hi, i am looking at a 50 that has been sitting for some time with out the spark plugs installed. the truck itself only shows 6500 miles on the odometer but with those holes open does it most likely mean a rebuild?

Well, likely a cylinder honing, possibly ring replacement- won't really know until you pull the heads, pan & inspect the bearings.

I'd say pull the heads, muddaubers, bugs, all kinds of things could be in the cylinders. But have you asked why it was parked to start with?