To start off, only Tier 5 items, such as [item_1hswordt5] can be a random legendary, and only if they are 600 Gear Score. Random Tier 5 items under 600 Gear Score will never be able to roll the last perk, meaning they can't be legendary. Note: We have noticed a potential bug where Tier 5 items at exactly 500 Gear Score fail to apply the restriction, meaning they can roll the last perk and be legendary.
Crafting 600 gear score items requires preparation, and well, luck. You're going to need to stack up all the possible bonuses, from the trade skill level, ingredients, food, perks, trophies and town projects. With all the bonuses ready, you'll craft Orichalcum items such as [item_1hswordt5] at a random gears core between 595 – 600 gear score.
However, there is an exception – Arcana – which can only produce 580 – 585 gear score items. Unlike other crafting trade skills, Arcana generally requires a quintessence like [item_essencewatert1], which compared to something like [item_ingott51], does not provide an increase in the minimum or maximum possible gear score. As of patch 1.1.0, Arcana gear can be crafted at a 595 - 600 gear score range.
Note on Arcana: You will want to use the highest tier available for the last material, such as [item_essencewatert1], which won't affect your gear score range at all. Using a lower tier one will decrease your minimum gear score.