340 or 360?

360 block was based on the 318/340. 340 was block was based on the 318. 318 was based on the 273. 273 was based on the 318 poly. 318 poly was based on the 277 poly that's in my 56 Plymouth.

Different bores, different main bearing diameters (360, all others same), other minor changes, but critical dimensions - bore spacing, deck height, length and width of the block, cam to crank distance, lifter bore angle and diameter, etc - are ALL the same, from 1956 until the third gen hemi replaced the A/LA. And even the latest hemi has the same bore spacing as the 277 poly in my 56 Plymouth.