Just a heads up that the 6k stack size modlet reduces stack sizes of gas cans(10k), dukes(20k) and old cash(10k).
while not really a big deal, it also reduces stack sizes of whats smelted in the forge(30k). that is kinda a big deal
I'm a total xpath noob, but i see your checking for a value greater than 1 to avoid things that should never stack, is it possible to also check for values greater than 6k?
-edit-
i seem to have gotten it working
feel free to update the 6k stack size modlet with this if you want.
<set xpath="/items/item/property[@name=Stacknumber][@value>1][5999>@value]/@value">6000</set>