Remember those cams are probably designed for a 350 so a 400 will tame the cam some. Something with a slight lope in a 350 will idle dead smooth in a 400. 400s need a larger cam to breath through 350 heads. Half a bottle of GM EOS at each oil change is way cheaper than a roller cam conversion. Keep the valvesprings reasonable, break it in right and flat tappets can last.