Depending on if you are trying to retain originality or not changes things. Original welding process was spot welding at random intervals but if you are not concerned with making it look exactly the same I think panel bonding might be a worthy option. Especially since you could lap seam the floor in the middle right at the edge of one of the stiffening ribs and just do bodywork along one of the grooves in the floor. It might be more fun than butt welding a 3 foot seam. If you can find a body shop with a spot welder you could take a picture of the original seam and have them run the spot welder around the edges to make perfect little dimples before you glue it down. Speaking as someone who has spent literally days removing spot welded panels I recommend cutting the floor out with a cutoff wheel, a light scuff with a 4” angle grinder to find the spots, hit the welds hard at a 45 degree angle to thin the floor without cutting up the understructure then peeling the trunk floor away with a pliers. It is much faster than drilling and it leaves a better surface to weld/ bond to.