The engine came with the truck and looks to have been there for a long time.
I pulled the fenders and got a better look at the block. It is indeed 3835692 and it does have the oil distribution plate I think. It looks like the picture in the Patrick's catalog. I don't have the indicated short oil line also seen in the picture. I do have an oil filter though.
The head has the shorter covers and two sources call it a 261 leading me to believe that inliners.org may have a typo. Did the 261 have two rocker cover studs? There are no perimeter bolt holes for the 4 bolt cover.