bb or sb for my 72 dart swinger

I'd go big block especially if your going the stroker route
440 has more Cid that a 408 and better bore and rod ratios and if you don't care about that stroke out the 440 to a 540 .
I can't see Big Block swap being all that much more expensive especially if your gonna run quality headers on either engine. Headers and aluminum intake get you close to factory Small Block weight and aluminum heads get you all the way there. What power range are you looking at ?