273 and 318 the same block?

A 273 can not be bored to a 340. You also can not make a 273 into a 318. In order to make a 318 or a 340 from a 273, 1 of 2 things must be done. Stroke it or resleeve it.
The blocks are all the same at a basic level. Internal water passages may differ somewhat.
It doesn't matter what or where the block came from. All blocks that seek a more than normal .030 size overbore should be sonic checked.

The only issue to making use of 273 heads in any other small block is the eay heads have different bolt angles and sizes.

If you want to build a stroker I would start with 72BNB's 318 block.
Or start with a 360 block. 340 blocks are to pricey IMO.
318's got to 390+
360's go to 400+ easy