[Mr70]

Those are the heads the LS-5 Corvette would have used,not a truck.
You might want to get the dates on them as well as the intake too.
It sounds like someone put this engine block in outside of the dealership.If it was a warranty swap,it wouldn't be displaying the TRK prefix,which,with that casting date,designates this was probably in a 1973 Chevrolet Truck at one time.Totally a coincedence that is also the abbreviation for the word Truck.The letter T is the important letter here.
T12 is a casting Plant control no. and is found on another engine blocks too.
What interests me is the engine casting date of B 19 73,yet the Tona code is B 13.(Can't cast an engine after it was assembled)..I would double check that to see if there is an error,or if it is correct,then it was assembled in one of the following years.

As far as trying to determine what engine this Corvette came with originally,if you can find the Radiator two letter prefix tag on the tank,relay it to me,as I can tell you what it's engine application would have originally been.
Also relay the Rearend 3 letter prefix,and possibly the Transmission information as well.
If indeed all these 3 components are authentic to this vehicle,then listen to what they are saying.