A stock 302 block can be a option with a main stud girdle upgrade.
If you dont, it will split right down the middle if you start making any decent HP. 350/400 is pusing it in stock trim.
My suggestion to you would be to find a seasoned 351W block, they are much stronger than the little brother 302, plus your starting out with 351 cubic inches, put a stroker crank in it and you'll have you a nice 400, then my friend, you'll be cooking with crisco.
I'd skip on the cleavland thought, getting hard to find them babies, plus finding good heads? forgetaaboutit.
Cp