The invention relates to a multi-part tunnel for a central underbody portion and to a method for manufacturing such a central underbody portion. The invention also relates to a method for manufacturing a central underbody portion (10) of the structural body of a motor vehicle, intended for defining the bottom of a passenger compartment of the vehicle, including a longitudinal structural tunnel (11) comprising the assembly of: a floor (12) of the central underbody portion (10); an integral central profile section (13) added to the floor (12), arranged in a longitudinal direction (X) of the vehicle and having a length (L) that is dependent on the length (D) of the central underbody portion (10) in the longitudinal direction (X); and at least one end part (14, 15) forming an extension of the central profile section (13) and configured such as to have an interface (141, 151) providing coupling and a function of taking up and transmitting loads with a structural portion (100, 200) of the body that is separate from the central underbody portion (10) and/or with said floor (12). According to the invention, the method includes a step of adapting the length (L) of the central profile section (13) in accordance with the length (D) of the central underbody portion (10) in the longitudinal direction (X) of the vehicle.