that first body line will hold the panle from moving alot from welding heat .having the new panel fit up will pay off a great deal. working the weld can help but less you have work with sheet metal alot you can add to a big mess . for dollys... part of a old leaf spring or a crow bar can be rework for a backer doly and body spoons use nail puller bars lifting door skins up to work then up you can make stuff up i have re worked bar stock to form metal. water will and can get in that panel and if it.s moister can get traped in threre if there no vents or drains