Is the 8th letter of your vin R ?
From the LMC link above:
2.8 Liter Engine, (VIN Code R) V6, 173 CID
1986 S10 S15 T10 T15 GCA, GCB, GCC, GCD, GCF, GCG, GCH, GCJ, UCF, UCH, UCJ, UCK
1987 S10 S15 T10 T15 FDA, FDB, FDC, FDD
1988 S10 S15 T10 T15 PCA, PCB, PCC, PCD, PCF, PCH
1989 S10 S15 T10 T15 TMA, TMB, TMC, TMD, TMF, TMH
1990 S10 S15 TMA, UJC
1991 S10 S15 FKJ, UJC
1992 S10 S15 XHF
1993 S10 S15 AAZ
This from a Fiero Forum regarding stamped identification numbers:
The last part of the VIN including some of the intermediate lettering is stamped on the block flange adjacent to the bellhousing of the tranny. The stampings on the cast iron of the block are barely visible and it would take some serious observation with the engine out of the car to determine the numbers.
The area is where the starter bolts on and it's on the top part of the casting.
The following is an excerpt from a GM shop manual.
All engines and transaxles are stamped with a partial Vehicle Identification Number. The stamping contains nine positions.
Position one is the GM car division identifier - 1=Chevrolet, 2=Pontiac, 3=Oldsmobile, 4=Buick, 6=Cadillac
Position two is the model year
Position three is the car assembly plant code
Position four through nine is the assembly plant sequential number of the car.
The serial # is stamped on the engine block directly under the thermostat housing just before the face for the timing cover.
This pic if from an 87 Fiero GT 2.8L
The last 3 letters are SCN which is correct for an 87 Fiero GT.
I believe the LMC codes are correct, but not the location. Look om the front
as in the pic above.
It's the same place they put the codes on most SBC's.
This also shows you what an ID stamping looks like as opposed to the casting mark that was pictured above. Casting marks simply ID which raw casting it is and is far from what the ID of the finished engine is after all machining is done.