The Form 1 is for building one specific rifle. After it's built it can change, but your application is to build it as the form is filled out. Pick the upper with the caliber and barrel length you want to use for the form. When the form is approved assemble the rifle with that upper, after it's assembled you can swap the other upper on anytime you want.
Make sure you put the total length of the firearm with the stock extended, so you will need to know what stock you are using and measure with it extended.
I would recommend scanning a pdf copy of your trust, make sure it is legible, but it's easiest if you keep it under 3 mb. Mine came in under 1mb.
When filling out the form use the model and serial that is on your rifle, if someone has entered a similar model, don't automatically use what is available - if it doesn't exactly match what your receiver has printed on it then don't use it. You can attach a picture of your lower that shows the correct original manufacturers markings if you want.