Small block strength

So were 360s actually overbored 318s with a longer stroke in an effort to save production costs or were they always different blocks? I know the 318 came first, but I'm just thinking by the roller cam era it would have been a 'common block' design.

A 360 is it's own block casting. They are all very similar just as all of the SB chevies are similiar but yet they're their own individual casting.