a starter and alternator later she's back on the road.
Make sure your tach is still accurate as GM's infinite stupidity used a larger diameter pulley on a lower RPM diesel to charge two batteries. A gas engine pulley on the alternator will read fast. You might be able to still get your core back and pulley swapped if needed.
There are several threads of using a larger and better alternator than the noisy firecracker edge of the thermal envelope GM abused their customers with.
Get under the rig and make sure the rear brace is still on the starter. Trust but verify because if it's missing it can snap the corner of the block off.
IMO this was a shotgun approach as both generally don't fail at the same time. Why not new batteries while they were at it?