The whole "numbers matching" deal started in the Corvette community, over 50 years ago. Chevrolet stamped a string of numbers and letters on the "information pad" in front of the rh cyl head, on the block. Engine plant, machined-on/build date, and letters to identify what the engine would be installed into. The letters in the suffix were model year specific, too. THEN, there was teh casting number and date on the lh side of the block, above where the oil filter area is. Plus a hidden casting date code on the bellhousing flange behind the distributor, about the size of my little finger nail. In later years, the last half of the VIN was also on the Info Pad, too, in a separate line of stampings.
On the B/RB engines, it is easy to determine if it is a 350/361/383/400 or 413/426W/440 by where the "info pads" are on the front of the engine. Defined in the factory service manual as to what the stampings mean.
IF you are concerned about "numbers match", you can ALSO check the stamping numbers on the distributor and carburetor! Provided they have not been changed. In many cases, unless theer was a big failure, they were rebuilt and re-used on the same engine as it aged.
1968 is significant as that was the year a HUGE initiative was launched to help stem auto thefts (and related "chop shops") in the USA. The last half of the original VIN was stamped into several other places on the body and the powertrain items. In those earlier times, only "The Fuzz" knew where all of the numbers were, other than the powertrain items. In later years, serious restorers have now found where the 'hidden numbers" reside.
Enjoy!
CBODY67