Sb/bb?

On small blocks;

The 318 is cheap and everywhere, but cubicly challenged. You'll have to think slightly differently on how it works best with what parts. There not a problem in making horse power, just how you do it is not the same as the larger engines. Good overall choice.

340, pay for the rarity of it. Can get bigger (Be biggest I should say) when stroked due to the largest bore.

360, Another dime a dozen like a 318. But the advantage is the longer stroke. For the price, which should be right about if not the same as a 318 out of a junk yrad, your gaining 42 cubes off the bat and a bit of torque.

With the larger stroke, you'll have more torque in the lower RPM range and peak HP comes a bit sooner. It can be stroked to within a few inchs of a 340.

This is the base as to which (I think) is a good perspective to look at the S/B engines to start with.

S/B vs. B/B is another can of worms. The S/B is lighter, easier to work on and slightly cheaper. Now flip it for the big block. LOL

Power wise, it's moving it's own weight, so that makes a weight issue a reason to go for one. Unless your going to town on a B/B where weight is not an issue.
Handling wise, the big block can become preety light, and if you go that route, it shouldn't pose a huge obsticle (sp) to get around.

I think upgradeing parts to handle a big block is a near moot point if the small block is also up'ed in power to a serious level. All the same things will need to be upgraded.
If left in a mello state (The S/B) you can be ahead of the game. ($$$)

To each there own, decisions decsions decsions....