The Falcon 9 Heavy will be SpaceXs entry into the heavy lift launch vehicle category. Capable of lifting over 32,000 kg to Low Earth Orbit (LEO), and over 19,500 kg to Geostationary Transfer Orbit (GTO), the Falcon 9 Heavy will compete with the largest commercial launchers now available. It consists of a standard Falcon 9 with two additional Falcon 9 first stages acting as liquid strap-on boosters. With the Falcon 9 first stage already designed to support the additional loads of this configuration and with common tanking and engines across both vehicles, development and operation of the Falcon 9 Heavy will be highly cost-effective.
Heck, just have them use the F-1 from the S-1C of the Saturn V. Those things will lift anything and are reliable (or they were). Not much design needed there.