The development of the Plus 8 was led by Maurice Owen, an engineer taken on specifically for the role. The Plus 8 prototype was based on a modified chassis from the Plus 4, altered to accept the Rover alloy block V8, purchased from GM-Buick in 1967. Plus 4's Moss gearbox was carried over and the Salisbury 7HA axle was uprated with a limited-slip differential. The chassis was developed in stages to accommodate gearbox changes in 1973 and 1976, the body widened in 1976 to accommodate the widened chassis and the wings widened to accommodate larger tyres to handle the increasing power and trend for lower profile and wider tyres. The original 1968 Plus 8 was wide and the last was (with an optional "widebody" at ) For several years in the 1960s the Plus Eight was the fastest-accelerating UK production car.