rule number 1: when crafting a Legend item, if you craft it in a way that it loses it’s Red border, it loses it’s Legend benefit. that makes it harder to make affixes maxed out. if an affix had a range from 5 to 100 on Legend items, when trying to max it out, the rolls would be 75 to 100. on items Normal to Epic, the rolls would be from the full range of 5 to 100. sure, you could get lucky on the rolls, but if not, that is a lot more Crystals getting the values maxed. if the item has a Red border when you add a Crystal affix or Mythic affix (which changes the borders to light blue and purple, respectively) it will retain it’s Legend benefit.
rule number 2: you wont be able to craft that Eternal like you want. Legends are used to craft Builds, Eternals have Builds crafted around them. umm, the only Eternal Weapon/Head item you could do something like this to are the Mythic Items (the ones with the Mythic Set on them). they have 4 Sockets you could make a Mythic affix with and can then add 3 Normal or Epic affixes.
just so you know, those DPS numbers on those weapons are based on the affixes that are on that weapon. if you want to know what the DPS is for the different Skills, take a look at the Stat Page. it will have a more accurate DPS based on most of the affixes from all the items that are equipped.