DO NOT WELD! You will warp the quarter panels, even if you do body work it will show some time down the road.
Fit the tubs to have a 1/8" to 1/4" gap from the edge of the tube to the inside of the quarter panel. Do not fit the tub tight against the quarter panel.
We use urethane seam sealer. Put tape on one side and apply a bead to the opposite side, allow to dry, then remove the tape and apply a bead to the opposite side. Trapping the edge of the tub between two beads is the strongest method.