i went through something similar on a Perkins engine I needed some parts out of for another engine. Claimed to have been very low hours, from the look over everything I believe it, but looks like no one ever changed the oil and it was essentially a varnish at this point, so bad I think it stopped running because the valves wouldn t close fast enough, had to beat them out with a hammer.
Piston rings were stuck in the pistons, no amount of prying, solvents, hot water tank cleaning would budge them. I ended up having to heat them with a torch to get the varnish hot and liquid enough to let go.
What vintage 268 is this? There s many flavors of pistons for the 268 or BSD4.4. Some have a dome in the chamber, some are flat bottomed. If memory serves me correctly I believe the later style pistons are only 2 compression rings and honestly that s all you really need. Some cat 3208 s only had one compression ring.

Mike, this block is the newer web block, the numbers are NH numbers (BSD 4.4) and it has the 9/16 head bolt holes.... that another change i have to deal with.. the pistons are all oem, standard size.. the story i got is it was in a 7710 and it had suffered from a cab fire.. everything on it looks like low hours. When I got it, it was missing the head and inj pump..

the numbers on the right side of the pan rail

F438883 3m14 87840243

so if i decode it right dec 14 93