For 2H weapons, 1 STR = 0.75 Attack. Also, you ignored the fact that Unkai Domaru +2 has Attack and GKT on it. Here:

* Unkai Domaru +2 has 17 Attack and 7 GKT Skill, which is +24 Attack total.
* Ace's Mail has 18 STR and 12 Attack, which is (18*3/4)+12 = 25 or 26 attack total.

So you should gain 1 or 2 attack when you switch from Domaru +2 to Ace's Mail.