Opinions on a Wrecked '70 340 DEMON

You don't need to crawl under it to see if it's a 340. Look on the front of the block where there red area is circled.

Whether it's the original to the car, then yes, you'll be under the pass side looking at the pan rail. The numbers you find on the front of the block may tell you whether it's necessary to even crawl under it...