YGK are STR 75%. No other mods, so just stack STR STR STR. If you're able to stack att, consider a ratio of 5:1 when comparing how much attack you need to outweigh one point of STR. i.e. 6 Attack should (very marginely) outweigh a single point of STR for YGK. Bear in mind though that all WSs use the extra fSTR mod, which you might guess comes from your STR.
Since YGK can't crit, you don't need to stack DEX at all. And since they're single-hit, you won't need to stack much accuracy either- though you shouldn't totally neglect it. Consider that hitting 9/10 WS's with a Hauby would do more damage than hitting 6/10 with Osode, for example.
Another thing to note is your WS hits also work with STP, though nobody really cares about it I guess.
Edit...
As for the legs slot, it goes like this in terms of WS equipment:
1. Royal Knight's Breeches
2. Myochin Haidate
3. Myochin Haidate +1
4. Shura Haidate
5. Usukane Hizayoroi
6. Hachiryu Haidate
7. Judge's Cuisses