I wouldn't say stroking is necessary, people pay big bucks to make a small block 400 plus cid.
400 vs 440 they pretty much have the same bore the main difference is deck height the 440 is really too tall for under 500 cid engine making the 400 better for sub 500 cid. A 451 stroker is basically a bore out 440 but cause of it shorter decks the engine weights less and less bobweight cause of the shorter pistons in a slight smaller package.
Do you need to stroke your 400 to make power. No you don't it's just gonna move the powerband up 600 plus rpm over a similar 440. But saying that it only cost a little more to drop a 440 crank into 383/400 making it worth while unless budget is very strict.