Easiest thing would to pick up a crappy m4 stock & figure out the length with it. You shouldn't ever be "stuck" with what is on the original form, the consistences from all the nfa boards is that as long as you can return to the stated configuration it doesn't matter what it's current configuration is. If you want that extra piece of mind, write the atf a letter notifying them of the change of configuration after you get your stamp back and finalize which stock/upper configuration.