Guy continued to be successful throughout the 1960s, with the development of the Victory trambus and the Big J series of trucks. The Big J was designed around a new, high-revving Cummins V6-200 engine coupled to a 12-speed splitter-type gearbox, and was intended for motorway operation. The reliability shortcomings and poor fuel economy of the Cummins V6-200 engine soon saw it replaced by engines from other sources. For example, AEC AV505, Leyland O.401, Gardner 6LXB, Cummins NH180/NTK265 and Rolls-Royce Eagle 220/265 were popular choices dependent on Gross Vehicle Weight. Gearboxes were sourced from AEC, David Brown and Fuller. Rear axles were selectable from ENV and Kirkstall Forge offerings. Such were the permutation of fitments that each vehicle's configuration was recorded on an "Erection Sheet" which, in the pre-internet age, was mailed from the factory to Guy dealers to have a record of each vehicle that may require attention in his workshop. The distinctive, non-tilt cab was sourced from Motor Panels.