Not familiar with a '55 but I'd suggest doing the replacement with the cab on the frame so you can check the alignment of any holes in the replacement panels with things like pedals, mounting bolts, etc.

Personally, I prefer to test fit and temporarily hold my panels with #8 sheetmetal screws. They are much stronger than clecos and you are not likely to break a #8 unless you use a power driver. I always use a hand screwdriver with a sharp Apex bit. Drill the parts with a 1/8 drill then drill the head side oversize to 5/32. If you do this you will easily be able to pull the two parts firmly together because the screw will not thread both pieces, only the one.

A MIG welder will work fine if you do overlap seams. If you are inexperienced it will be nearly impossible for you to weld a butt joint. Be sure you clean ALL of the rust from areas to be welded or you will play hell getting a good bead. Rust will cause the beads to bubble and pop, blowing holes in the bead and maybe even the base metal.

Tack the panel in place with single spots and skip around when you finally start running beads. Keep the parts cool to avoid warping. A wet rag or blow gun work well.

Be patient and careful not to cut yourself...

Good luck.

Woody
Your Brother in Bolthood

My 1951 half-ton 'Ol Red

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