Sooooo ....  When we attempted to reassemble the crankshaft in the block, our first step, naturally, was to insert the new main bearings we had ordered.

THEY DIDN'T FIT!!!   What the ...  ???!!!!?!?!?!

Because the casting number and the lack of Captain's bars told us this was a '54 (AKA "Early") 261, we had ordered early style main bearings (with a placing nub that protrudes part way around the outer circumference of the bearing and locates into a corresponding hole in the main journal of the block.

*This* engine, though, was fitted for later style main bearings (with the tab/slot on the edge that fits into a corresponding slot on the edge of the main bearing journal (see the image below -- the slot is visible to the left of the drill bit).

Whiskey Tango Foxtrot, over?

Well, remember the weird mismatch on the casting and date codes?  We didn't ...

Main Bearing Mis-Match

Apparently, this engine was built to be a replacement engine for an "Advance Design" application.  Thus, they used the early mold ('54-'57) for the block to ensure the cooling fan (among other things) would be in the proper/higher location for installation in a '54 or '55 1st Series truck (Advance Design) needing a new engine.

BUT, because they built it in 1959, they upgraded the main bearings to the more modern style because that's what the engine assembly process was set up for (final application didn't care what style bearings were used ...).

Fine.  We figured that out.  But ...  We were sitting there, all ready to reassemble the engine, and we were at Full Stop because we had the older style bearings.

And we had a shop full of students standing there waiting to learn the next steps in engine rebuilding, so reordering the main bearing set and waiting a couple of days for delivery was out of the question.

As Pooh Bear would say whilst tapping his forehead, "Think! Think! Think!"