I can also vouch for the originality of the 68 383 muffler to head pipes being welded from the factory. One car being a October of 67 production and the other being a July of 68 production. I would think as mentioned earlier, it was a cost thing. Also, being that it was not a "H" pipe, the muffler and pipe could be positioned in a jig for quick welding and no real issues with the installation process.
I have been fortunate to view the warehouse, housing many of the obsolete parts through the years that Frank Mitchell accumulated and assembly line pieces were there for purchase as well. Just like any aspect of these high end restorations, it is the details that separate the upper levels in the pursuit of assembly line perfection.