What to upgrade next? (in General)

An Amulet of AC [0] (+18) 1,664,312
An Adamantite Cuirass [40] (+71) 4,627,325
A Shadow Cloak [11] (+20) 1,238,978
A Set of Chain Mail Leggings [18] (+39) 2,475,509
A Set of Tulkas' Gauntlets [6] (+18) 2,982,926
A Helm of Durin [9] (+22) 3,206,747
A Mithril Shield [22] (+45) 2,264,711

CB-T [1] can't be upgraded.
Total AC 340
Total AC needed 400

So I still need 60 AC, which item should I upgrade next?
Keep at it with the adam? Or should I start forging the shadow cloak?

Whatever costs the least to up one point. Upgrade one point. Lather, rinse, repeat.

I think AC can be upgraded a bit more cheaply, and also take into account that some armor like AC can be forged cheaply whereas Amulets would usually mean blacksmithing.