The 426 Max Wedge is a race engine that you could order across the parts counter as a complete package. Cross-ram intake with two Carter AFB four-barrels, lightweight magnesium intake on later cars, factory-tuned headers, special pistons rated either 11.0:1 (415 hp) or 13.5:1 (425 hp). It is not the 426 Hemi — that engine arrived for 1964 in NASCAR form and reached the street as a 1966 option.
Plenum carries the Max Wedge as an encyclopedia entry rather than a decode option because no production B-body broadcast sheet shows a Max Wedge engine code in the regular cells; race cars were essentially special-order, and the documentation set is its own corner of the Mopar reference universe.