An improved modular heat exchanger suitable for automotive applications, and particularly radiators for heavy duty equipment, and methods for forming the modular heat exchanger. The modular heat exchanger construction incorporates a brazed core assembly composed of flat-type cooling tubes and sinusoidal centers. The ability to use a brazed core assembly within a modular heat exchanger construction promotes enhanced heat transfer efficiencies associated with the use of flat tubes and sinusoidal centers, as compared to mechanically-joined round tubes and fins. The required positional tolerances of the tubes for mating with the remainder of the heat exchanger are maintained within the brazed core assembly by eliminating core shrinkage attributable to the use of clad aluminum alloy components to construct the core assembly. In two embodiments, core shrinkage is physical suppressed through the use of an expandable brazing fixture or the inclusion of sub-headers in the brazed core assembly. In a third embodiment, a clad slurry containing a braze alloy is deposited prior to brazing on a core assembly composed of unclad tubes and centers.