Hi Lon,
While ‘cladding’ a partial 'repair panel' over an existing panel and ‘blending it’ into the body line may seem like a lot of work, that cosmetic repair is more easily accomplished than ‘doing it right’.
Full replacement of '55-'57 'Bird inner and outer rocker panels is no easy task. They are truly ‘built-in’ to the body structure. Originally:
-the inner rocker is spot welded to a tab on the passenger compartment floor pan
-all of the passenger compartment floor brace ‘boxes’ have tabs that are spot welded to the inner rocker panel as well as to the floor pan section
-the inner and outer rocker panel are spot welded together all along their top and bottom edges
-the front end of the O.E.M. outer rocker panel slips into a sheet metal pocket that is part of the bottom of the door post assembly. My recollection is that the front end of the outer rocker is spot welded to that door post 'pocket' it fits into
-the rear end of the O.E.M. outer rocker has an end 'block-off' piece with a tab that also was originally spot welded to the inner rocker.
The O.E.M. inner rocker panel is 16 gauge steel. The O.E.M. outer rocker panel was 20 gauge steel.
Structurally, the 'box' formed by the inner and outer rocker panel on a 'Bird, and the connection of that 'box' to the floor of the passenger compartment, the front door post, and the rear door jam are critical to the structural integrity of the body.
While not referred to as having unibody construction because of having a full frame, I can’t think of a major sheet metal component of the basic ‘Bird body shell that was not spot welded together, likely to give integrity to the body because of it not having an integral top. The seams where the outer rocker panel meet the front fender and rear fender dogleg were leaded.
Appropriately installed and aligned replacement inner and outer rocker panels on a ‘Bird are to be appreciated and admired, IMO.