So, Arcane Action states that “Success indicates that the wizard has passed the test as if he had been testing the appropriate ability.” This rule, written as it is, heavily implies that the sorcerer performs an actual physical task, which is guided by their magical prowess.
This implication is supported by a example text: “Using this, a sorcerer may strike an unsuspecting opponent or climb a wall.”
Following this logic, Arcane Action can allow a wizard to script actions in Fight and substitute their Sorcery in place of the required skill. So, yes, Block instead of Cast Spell would absolutely be completely valid here.
As for weapon length, that depends on what you are wielding. Arcane Action does not grant you a weapon, it only allows you to magically wield whatever is already on hand. So if you’re unarmed, you’re fighting at your natural reach, and you’re probably testing Sorcery as if it were Brawling or Boxing. If you’ve got a long sword on hand, Sorcery is a proxy for Sword, and you’ve got a Long reach.
Does that make sense?