72 dart full floor help

Is it possible that the car sagged a bit when you removed the old floor pan? I don't see any bracing on the car to prevent any flexing. That is a lot of structural metal to remove without bracing it up first. Try raising the front of the car off the ground, as evenly & as carefully as possible, & place jack stands under the front frame rails, that way the front end might flex enough the other way, so the gap is tighter when you weld it in. Just an idea. Some cars were pretty poorly assembled though from the factory too.