Vonnie Holliday is probably stronger than Manuel Wright, at Holliday's current weight. And, I say this because I've researched it and discovered that Holliday is indeed abnormally strong. In Green Bay, he was the strongest man on the team, even while Gilbert Brown was around. At 6'6", Manuel Wright may be a little tall to play the NT position, not enough leverage.
And 30-40 pounds is doable, if you don't mind the fact that it will mean Holliday will lose a lot of quickness and endurance. If you were asking a dude to pack on 30 pounds that fast in a way that he won't lose too much speed, endurance, or quickness, and will just gain a ton of strength, that would probably be impossible. However, Holliday has the strength, at NT he wouldn't need a whole lot of quickness, so what he really needs is leverage and mass.
Holliday use to be a 300+ pound DT in college. I just don't think his future with this team lies as a passrusher. He's become a run stopper. He is signed for 2 years. Let's see if he can put on weight and become a decent NT.