I did all mine as flange joints, then read I should have done butt joints. The theory was that parked in the sun, the thicker metal in the flange area expands at a different rate and might then show up. I haven't seen it happen though, so it may be less of an issue with these old heavy gauge fenders. Another problem of flange joints is that they can trap water and rust, so the seam needs good sealing. A flange joint is readily noticable from the backside.
Flange joints are more tolerant of cutting errors and are easier to clamp & weld. Butt joints take more skill - you have to be able to get the joint uniform, need a
gizmo like this to hold the panels while welding, and have to be more careful about burn through, but the joint will be nearly undetectable when done right.
