400 block
if you are stroking it the kits come with rods/pistons/crank
thus bob's answer
that being said just 400 c.i. will put out a lot of power if you are building it right.
if it's racing get 12:1 pistons run race gas get some good heads on there
if it's street get 10:1, get heads if you can afford them as well. if you cant get heads they will work but will be your limiting factor
if you do go stroker it's nice but make sure you get the right pistons in your kit if you are running race gas or pump gas it matters the most.
i'm not sure if it would be better to get good heads and skip the stroker kit or get the stroker kit and use the limited head flow.
flip a coin
if money was free you could get both
if you are racing definitely get race heads