I know that in our modern world, we want things which are "drop in", fit, and work flawlessly wiith a minimum of effort. BTAIM
My point is that as with the black panel, it looks really good, but if the dimensions are off (or don't match), it's going to take effort to make it work and fit. It'll at least need some extra pieces and such in the illustrated vehicle. EACH one will need a good amount of structural adhesive and sealer before they are ready to see underlayment and carpet, either way.
Structural adhesive? Yes, the same stuff that glues the roof panel on later-model vehicles. With a few screws/pop rivets for good measure. Then a coat of good anti-rust paint on the underside, covered by a good layer of undercoat.
To me, one thing which seems to be in C2C;s favor is that it is more universal in nature. Which CAN cover some of the little unspoken-of things that can happen in the body jig set-up phase prior to welding (in the body shop section of the factory assy line). For example, the roof drip rail on my '70 Monaco 4drht is normal on the lh side, but minimal on the rh side. Roof was not installed squarely, but everything else fits and works well. These are things which will not become apparent until "you are there".
The conversation about "building your own" is valid, but if the C2C is over half-way there, then that can make it a good place to start, without having to buy the material, bend it, bead it, etc. Crude it might be, but better in other ways, because of it. Better than using fiberglass, like in "the old days".
Several ways to look at it. Everybody has their own sensitivities in that area.
Happy Holidays,
CBODY67