Rover saw Buick's compact 3.5 L V8 from the Buick Special as a way to differentiate the P6 from its chief rival, the Triumph 2000. They purchased the rights to the innovative aluminium engine and once it was modified to allow its use by Rover, it became an instant hit. The Rover V8 engine, as it became known, outlived its original host, the P5B, by more than thirty years.

The 3500 was introduced in April 1968 (one year after the Rover company was purchased by Triumph's owner, Leyland) and continued to be offered until 1977. The manufacturer asserted that the light metal V8 engine weighed the same as the four-cylinder unit of the Rover 2000, and the more powerful car's maximum speed of 114 mph as well as its 10.5-second acceleration time from 0–60 mph (97 km/h) were considered impressive, and usefully faster than most of the cars with which, on the UK market, the car competed on price and specifications. (The glaring exception was the Jaguar 340, which was substantially quicker than the Rover 3500 and was advertised at a 15 per cent lower manufacturer's recommended price, the Jaguar representing exceptional value as a "run-out" model, to be replaced later in 1968 by the Jaguar XJ6.)

It was necessary to modify the under-bonnet space to squeeze the V8 engine into the P6 engine bay: the front suspension cross-member had to be relocated forward, while a more visible change was an extra air intake beneath the front bumper to accommodate the larger radiator. There was no longer space under the bonnet for the car's battery, which in the 3500 retreated to a position on the right side of the boot. Nevertheless, the overall length and width of the body were unchanged when compared with the smaller-engined original P6.

Having invested heavily in the car's engine and running gear, the manufacturer left most other aspects of the car unchanged. However, the new Rover 3500 could be readily distinguished from the 2000 thanks to various prominent V8 badges on the outside and beneath the radio. The 3500 was also delivered with a black vinyl covering on the C-pillar, although this decoration later appeared also on four-cylinder cars.