Overload doesn’t change when you can cast the spell.

If you don’t pay the overload cost of a spell, that spell will have a single target. If you pay the overload cost, the spell won’t have any targets.

Each token copies exactly what was printed on the original artifact creature and nothing more (unless that creature is copying something else or is a token; see below. It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

If you are instructed to cast a spell with overload “without paying its mana cost,” you can’t choose to pay its overload cost instead.

If the copied artifact creature has in its mana cost, X is 0.

Casting a spell with overload doesn’t change that spell’s mana cost. You just pay the overload cost instead.

Because a spell with overload doesn’t target when its overload cost is paid, it may affect permanents with hexproof or with protection from the appropriate color.

If the chosen artifact creature is itself a token, the token March of Progress creates copies the original characteristics of that token as stated by the effect that created that token.

Any enters-the-battlefield abilities of the copied artifact creature will trigger when the token enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

If the chosen artifact creature is copying something else, then the token enters the battlefield as whatever the chosen creature is copying.

Effects that cause you to pay more or less for a spell will cause you to pay that much more or less while casting it for its overload cost, too.