Crummy, FWIW - You engine could be a conglomeration of parts from just about anywhere as these old trucks of ours have been fixed and/or refitted with sometimes what ever was handy to get them back on the road. One thing that may help lock down the engine ID detail is to locate the casting date codes on both the block and head. For the block, those should be behind the starter mounting location on a level with the casting number you found and reported above. On the head, you will need to remove the valve cover as the date casting code info is on the top surface of the head just inside the valve cover sealing surface.
Find those tidbits of info and post them here and we can try and pin your ID's down a bit better. On the stamped "KBA..." serial number, it is possible that those numbers were stamped by a shop that may have installed a replacement service 261 engine (they came from GM without any serial#s stamped) to match the original engine serial number that it was replacing. Reason that may have happened is most trucks of these vintages where registered/titled using the engine serial number for the vehicle ID number instead of the cab serial number, so a replacement engine may have gotten stamped to keep the original title information the same. With the possibility of many previous owners and shops working on the truck, it's very hard to determine how it may have actually happened?
