Martin: Check
this link on ID Plate locations on my website and then report back on the engine serial number.
GMC engine numbers will often, but not always, begin with a letter which denotes the bore and crank journals other than "standard" and then three numbers for the engine size, which in your case would normally be a GMC 228.
Sooo....
If no code letter then the engine bore and crank is standard. IF it has a letter, the following apply:
LETTER..........Cylinder Bore Crankshaft Journals
...............................Main.......Rod
A .............+0.005".....-0.010"....-0.010"
B .............+0.005".....Standard...Standard
C .............+0.005".....-0.010".....Standard
D..............+0.005"......Standard...-0.010"
E............Standard......-0.010".....Standard
F............Standard......-0.010".....-0.010"
G............Standard......Standard...-0.010"
Hedge: I have never seen a GMC motor that didn't have a number on it when it left the factory. Every GMC engine made carried a serial number and the 248s were stamped between the flywheel timing opening and the distributor mount on a flat spot facing up on the block. Its possible that your's was ground off and I am curious to know if your flat spot shows any sign of being ground down. Is this spot level and flat or is it by chance pitched away from the motor on a slight angle?
I have to beleive that your number was ground off by the chop shop that stripped stolen truck and then sold you that motor cheap!
