440 what am I missing here?

All the more reason to switch to an Edelbrock performer rpm. Optimal rpm rating of 1500-6500 (keyword is rating, YRMV) as opposed to the 3000 to 7500 rpm rated manifold. You had also mentioned the possibility of fabricating headers for it. I’m curious if Andy thinks it would be worth the trouble and if so what size & length of tubes would work best.