22 years ago, I cut off much of the bottom of this badly rotted '55 Courier including rear quarters, inner and outer rocker panels, all the floor panels and floor risers and bought mostly cheapo repro panels. I ended up regretting that. EMS appears to have the best quality and acceptable metal thickness.
I suggest not using the tall rear quarter panels if you can help it. The shorter version available is cut below the taillight.
I used a very cheap pneumatic metal flanging tool from harbor freight for all my seams and it was the best investment I ever made in auto body repair tools.
After all welding is finished, try to get behind the weld seams and brush in some sealer into the seams. I brushed in self-sealing primer, then overcoat, then on top of that, I caulked the seams with 3M Fast-n-Firm.
I did not cut off the bottom of my doors, but cut out and made patches to weld in. EMS makes both inner and outer bottom of door patch panels that appear of real good quality.