It is not advisable to use 305 heads on a 400, even the 416's, if you look into the intake ports you will see the small "window" that these heads have, that along with the smaller valve is done intentionally to give the 305 some throttle response and keep the intake flow moving, you will also have a very octane sensitive engine unless you keep the quench area right at .040, pitting big valves in these heads is like putting a 36" screen door on a 30" door opening, what have you gained? also when drilling steam holes in the heads you must use a slight angle towards the center of the head. I do these every week, I try to direct customers a different way but I won't try to loose the sale.( For the money you invest in these junk heads you can get aftermkt that make 30 hp. more stock right out of the box), I gotta eat too! and the customer is always right.