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The number your wife found is the block's casting number. It indicates that this block was from the first series of 360 blocks cast from 1971 to 1974. These blocks were cast using 340 cores so the cylinder walls are thicker and should be able to withstand a 4.070" bore, with sonic test up to 4.100".
So I don't think the engine is a '76.

You don't know what's in the engine unless you research the numbers or built it yourself.

Now you need to find the engine ID number. Stand at the front of the car, looking directly back in the direction of the steering wheel. You're on the driver's side, your knees are against the bumper.

Look at the valve cover, follow the surface down the front edge of the head until you get to the gap where the head gasket is clamped between head and block. Now go down about another half-inch. The Engine ID number is stamped here. The numbers aren't very big and may be obscured by paint.
Let's suppose the engine is a 1972 360 built on October 31, 1971. The engine ID number will be something like this:
HM360R1031xxxxE.
H = 1972 model year
M = Mounds Road engine plant
360 = guess what?
R = Regular gas, or P means Premium, or it may be missing entirely, I cannot remember.
1031 means Halloween
xxxx is the engine sequence number, for that day
E means it had a cast crank, all 360s have cast cranks.

So what does yours say?

R.

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The camera is useful and with a scraper on my newly painted engine I got this: The first letter is very vague but it could be a G, then W360R35270347. No letter after the last 7 though.