I would not waste your time with stock heads. The aftermarket heads not only flow better they have a much more efficient combustion chamber and are aluminum.
That poor 408 will peak power at 4000rpm. I guess if you’re building a low rpm engine then by all means use a stock head.
You would be better off sinking money into a good set of heads then dropping money into the stroker assy for a 351 and skimp on the top end. If your going to build something don’t go half way, build it right the first time. Instead of down the road wishing you did that or this different.
Also I caution you if you stay with a cast iron heads if you hurt it with the nos, it is much harder to repair and more expensive then the same repair on an aluminum heads.
Scott