Seam welding is a a job which can take days. To do the thing properly all the underseal and sound deadening must be removed. I am lucky and have a rotating spit to mount cars onto and can spin them around to do things more easily. I would'nt recommend doing it unless you are really committed to going really hard in your Datto. I would reweld around your leaf spring mounts and down the strut towers and run a brace from the end of the sill to the underside of the strut tower.You can strengthen around the front tension rod mounts as well.A strut brace which bolts on( I've seen drongoes weld them in!!)helps to stiffen the engine bay box. Fit a well engineered rollcage that connects all the suspension pick up points and tightens up the shell. If your really keen you can convert your rear leafs to a slipper type arrangement, that is the rear spring eye is cut off and the spring leaf is retained between some bronze or alloy rollers. This helps stop the springs winding up and steering the rear of the car. Most of the sucessful coupes have had their rear shocks turreted (mounted straight up & down) as this is the most efficient way for a shock to work and you can fit longer travel ones as well. You can then tie the top shock mounts into the flash new cage you will be building.