Your firewall and trunk pan should keep the rockers from moving outward. But I wouldn't let it sit like that (without a floor pan) for two months or anything.
Take a measurement from two given spots on your rockers. Just prior to welding in your floor check that measurement to be sure nothing has moved.
It will be somewhat difficult to install the pan with a temporary brace across the floor from rocker to rocker. If you have any movement the rockers can be cinched back tighter by twisting a tow strap between the two just prior to welding.
Just make sure all of your floor pan tweaking is done prior to running the first tack weld. Speaking from experience, the Dynacorn pans need a bit of work.
