The bodywork of the Montreal was assembled in the Bertone factory at Caselle (a secondary factory to the main one at Grugliasco, situated near Turin)
Alfa Romeo (Arese factory) sent the floorpan to Caselle along with the individual components body components. Much of it was identical to the Giulia GT (105 series), apart from the firewall which was prestamped with the chassis number. There were a few assembly lines set up for the framework of the sides, the roof structure, and the front and rear ends.
After assembly the body was sent to be painted and trimmed, to the principle factory at Grugliasco, where it was prepped for electrophoretic coating. First grease removal and then etching. The last is a chemical cleaning dip in a tank containing a special solution.
It then proceeded to the phosphatising of the panels to protect them from corrosion. Next up the body was then immersed in an electrophoretic tank to apply a protective base paint allowing a coating in all the hard-to-get-at areas. Then the application of sound-proofing on the floorpan.
The final paint application was sprayed by hand rather than machine. After that it was final body assembly before being sent to the Alfa Romeo Arese factory for all the mechanicals to be assembled.