[C10 - C90 Bill]

For the Engine Suffix Code:

F = Flint Plant

05 = May

25 = 25th Day

WA = '65 and '66 (283 w/Manual Trans)

Looks like you do have the Original Engine in there. And the Online Sites were Wrong, lol.

They stated the WA was '65 and not '66, but my Parts Books shows '66 as well.

Also, you will notice that the Date above does not include the Year. For that you would need to check the Date Code, but it is on the Back of the Block and hard to see with the Engine still in the truck.

But I would be pretty confident that it is the original '66 Engine you've got there.