To the OP:

Your assumption that later slant six blocks would have thinner walls isn't correct. There seems to be no best year for blocks, if you are going to spend major bucks on a block I'd sonic check it first. And because Slants are bore-limited, I'd look for one that could be bored out more if your are going NA.

BUT, from my point of view, the easiest way to get a Slanty into the 11s is with a turbocharger. And that will work with stock displacement. Looking at the engine's layout, it seems made for a turbo, plenty of room for everything.

There is a lot of info on the Web, just start doing searches and you'll come up with plenty.

R.