Do some research and reading in your spare time (not meant sarcastically).
Check out Chevrolet small-block engine - Wikipedia, the free encyclopedia
The 283 was an overbored 265. The 327 was a stroked and overbored 283. Hence the reason for the early small journal 327's. I think GM realized they could restroke the 4.00" bore 327 to 3.48 without any block clearance issues, so why not take advantage of the longer stroke and rod to stroke ratio and build 350's. I've read in several places (not known to be fact) that the 307, that came out in '68 was the result of alot of 283 blocks 'laying around' at GM that could use the newer 327 cranks. And, again from what I've read, the 302 was a response to racing displacement rules. Now, for the 400, I don't know, maybe GM wanted lead the pack for small blocks.
Of course this is my opinion.
But one thing that GM did was maintain a very similar platform that led to alot of interchangeable components amongst different displacements and model years, which is real good for us and made the SBC very cheap to build. For example, check out the pricing on building a stage II or stage III 455 Buick or hot 340 or 383 Mopar.
AGAIN, just my opinion.