Moving to the new panel, check measurements again before cutting pillars etc. for jointing. At this point it is probably safer to leave a "safety margin" of about 1cm (in the right direction!!) when cutting any mid-panel joints. When the panel is offered up for a trial fit and clamped correctly in place, this overlap can be marked and removed more precisely, or crimped with a flanging tool so that one panel slots under the other. Clean up all surfaces to be welded, drilling spotweld holes in the new panel if required. Refit to the vehicle and adjust so that all panel gaps are correct and window openings aligned.
Clamp in position and weld all points that are required. Butt joints across pillars or sills must be seam welded to regain their original strength. Weld short stretches to avoid excess heat build-up and warping. (Tip - spot weld a thin metal strip behind wide butt joins to help prevent burn-throughs
After all welding is completed, remove excess metal from spot and seam welds before finishing as normal with body solder or filler. Solder is still commonly used on car production lines to fill and hide similar joins. Welds across pillars or sills will probably need to be tapped down slightly after grinding, as the welding process will almost certainly have created some warping, leaving high spots which will protrude when the filler is sanded down. Take extra care around window openings, as tags or bumps may prevent windows from sealing properly, or even cause them to shatter. Joints should be resealed with seam sealer to prevent water leaks and wind noise.
On completion of all filling and sealing work, the vehicle is prepared for primer and painting as normal. Don't forget that some interior paintwork damage may have occurred around welds, which will need rectification, even if covered by trim, etc. Completed repairs should be treated with some form of rustproofing to panel interiors
