im still gonna vote BB, things like trans, rad, would need to be upgraded anyways if you built up a smallblock.
I've been building on a 340 for a decade, as you can see in my sig. its a hot runner for a street car, but im on my 3rd camshaft, 2nd set of heads, 2nd trans, 2nd rad....etc...etc...basically everything i had when i started has been replaced be upgraded components to keep up with the motor..withthe exception of the crank/rods and pistons.
And guess what? I still want to go faster (its natural to feel that way)
Truth is, had i started with a BB i would have spent less, and most likely my car would be faster today.
Last summer i picked up a virgin 400 bb off a member here and with it came all acc. brackets etc. heck, he even threw in a 727 core for me.
what i paid was very similar to what i woulda paid for a smallblock core...and in the end i'll have much more motor (500"ci) which will put my current 340 in the dust, and 95% of the 408" motors in my taillights as well. i figure all said and done, this is gonna cost me the same as what i spent on the small block over the years, after all my parts swapping tryin to go faster.
The Moral of the story is to not fool yerself with what you want and what you are building. When i built my car it was a daily driver, which ended in its 2nd year as it was too much stress to park outside at the mall etc.. then again, had i built a 500"ci motor off the bat...i might not be here today writing this...so just be honest with yourself, a 400hp motor in a 4000lb car isnt exactly gonna put a hurt on any newer gt stang, and you will most likely will be lookin at the new challengers taillights should the occasion happen to arise....