360/300 magnum Questions/suggestions

Look directly at the front driver side of the engine IMMEDIATELY under the parting line between the head and block. There should be a long stamped number under there, and the first few numbers should have 318, 340, 360, etc right in that number, UNLESS it's some factory re-man, some are "xxxx" 'd out. (I have a "Mopar" branded reman 360 that's this way. It had a "Mopar" id sticker epoxied to the block)
Then you'll have to get underneath and look for the casting number on the side of the block.

What you have there could be anything from a 273 to a 360