84S10 -
You can find the year of the engine from the casting dates on the block and heads. On the block, the code is on the pad next to the distributor. On the heads, the codes are near the area of the exhaust manifolds. The heads will be easier to see, but they may not be original to the engine.
Code breakdown is as follows: Letter - two digits - one digit. The letter indicates the month, the two digits indicate the day, and the last digit is the year. This continued through the last 400ci T/As. Example B099, would be Feb. 9, 1969.
Once you determine the year, then look up the block code. You can also look up the head code which is two numbers, or a number and letter code on the center exhaust ports. That may verify if those are the heads for that engine.
One last thing is the block part number which is also near the distributor. If all else fails, you may be able to reference that (though it usually never comes to that).