If the big dollar default happens, and they go back to gold and silver money, it would be best to make the fresh start with metric units like gram. Well, kg as well, but most people wont need that very often...
It is much easier to use metric units instead of fractions of ounces or pounds or whatever. Its also unneccessary to print a silly dollar value on coins, which has no relation to reality - just as it is today already.
A gram of silver is worth a gram of silver, and gold is gold. And that is a constant, it has nothing to do with a fiat currency of any kind. The only variance is how much you can buy with it, and that is stabilized by the limited supply of the metals.
Thats the whole point of it: to have money that cannot be corrupted by banks and governments and not endlessly multiplied.