Engine numbers

The "good" news is that prior to 68, "date correct" and "matching numbers" engine are the same thing, so all you have to do if you're really concerned, is find a date correct 273. The 318 might be a good thing as it's more cubes and easier maint.

If you bought the car under the assumption that it was the original engine, then that's a completely different story.

I'd pull the valve cover and see if it has adjustable rocker arms.