[jocko]

My 2 cents, for whatever that's worth...

Concur wholeheartedly with FirstOwner, and I'll add a little.
As he outlined, the V113.... stamping on the protecto plate and the block should match to be "numbers matching".
The build sheet will only provide a way to DISprove engine matching number status. Note that only the 3 digit code for the engine is included on the build sheet - and several vehicles would have been manufactured with this 3-digit code (TBC on the attached build sheet), including the truck in question, again if it is numbers matching. So, presence of a build sheet with a 3-digit code that matches the stamping of your engine means it COULD be a correct motor, but doesn't prove it is. It just enhances the chance it is correct. But if the 3 digit code does NOT match between build sheet and engine block, then you KNOW it is not original.
The protecto plate, as FirstOwner posted, further narrows the field toward certainty. Or, at least as certain as you can be without the actual VIN being stamped on the block, which, as was pointed out, didn't occur until later.

so, in 67-72 trucks, my pecking order goes like this for engine originality:

Best case: Protecto-Plate in hand that matches the VIN and that code matches the one stamped on the block pad PLUS build sheet in hand with matching 3-digit code on the block casting.

2nd Best case: Protecto Plate in hand and matching, but no build sheet (I'd personally still call this "numbers matching" as the Protecto Plate is more discriminating than the Build Sheet).

3rd Best case: Missing protecto plate, but you have the build sheet and the 3-digit code matches. In this particular case, all I could honestly say, as a seller is that the engine COULD be correct. At this point, I would begin to reference the engine build date, etc to narrow down the possibility of the engine in question being in the truck - if dates are kosher, it could honestly be called "date-correct, possibly original motor". So, let's say someone dropped in an "era correct" 72 350 in their 72 - it could possibly have an engine build date on the block that occurred after the assembly date of the truck - proving it was not the original engine... If the engine were assembled before the truck assembly date, then it could be original. Etc etc.

If none of the above 3 are present, then it can't be PROVEN to be the original engine.
If any of the above are present and NOT matching, then it's not the original engine.

So, having only the build sheet doesn't prove correct engine, only the possibility of it. However, if I were buying one of these trucks, I'd be happy if the 3 digit code on the build sheet matched - even if the protecto plate were missing. These things weren't bought as collector vehicles in 67-72, they were purchased as trucks and as they changed hands over the years, it's a miracle any protecto-plates survived. If you have the pp and build sheet, you are a lucky owner - and that's the only way to really call something a verified numbers matching vehicle.

In other words, when you run out of ways to DISprove an engine is original, it's original. In later years it became possible to positively confirm lineage without any paperwork when the VINs were added to the blocks. Boy, wouldn't that have been nice!?