"Also you would only worry about the inside hardnesses?"
Interesting question; is the tank shell steel hardenable? Of course it isn't. Will you melt the steel on the interior? Nope; that would give a leak. Thus = impossible to harden the interior of the tank.
Why use 7014; 7018 is the standard for a malleable & strong weld deposit? Run a small mock-up coupon, and section the weld. The Vickers hardness traverse needs to include Shell basemetal, shell HAZ, weld root, weld interior, weld cap. Might as well include Hot-tap nozz hardness too. IMHO, any hardness below [BHN = 200] is acceptable, and there are arguments for harder welds being OK.