New to 340 so I have questions

Yes the number confirms it is definitely a 340. The -8 indicates that it was the 8 time that the casting mold was used.

I am not sure what the numbers before the 340 mean.

The casting date is on the passenger side under the #2 spark plug.

Blocks cast in 67,68,69 and part of 70 were candidates for the X heads. In 71 the J head was introduced. I read somewhere the j heads were used on 340 six packs with 2.02 valves.

Here is a thread about the heads.
X-heads vs J-heads: what's the difference??