To find the info on the block you need the casting number.
The casting number is located on the REAR of the block.
You then you go to www.mortec.com. They have a little reference list that tells you what you want to know.
To tell you if you have the right part, well a part number and mnfg. would help. Without it I have no clue what you hold in your hand, and frankly, I'm afraid to ask.
The L designation could mean for a 350 late block that had a roller cam or it could stand for the VIN for a (IIRC) L48 engine. I'd find that hard to swallow though as the aforementioned late 350 block had a designation of L98 in the F- and Y-body cars (same block but no roller cam in trucks and vans for a few years) and that the same chain for the L48 is the same as the L65 and L82 and just about all first gen 350 blacks through '86 I beleive. Part number, or go to www.advanceautoparts.com and just look up the info from the block that you got by going to mortec.com. You did do that eh?
Now, it could also be just a dumb designation from the mnfg. You could go to their website a peep through to see if you could find anything there.
Was that useful enough?