The Road Runner was based on the Chrysler B platform (the same as the Belvedere and Satellite), as a back-to-basics mid-size performance car. The Road Runner of 1968-1970 was based on the Belvedere, while the GTX was based on the Sport Satellite, a car with higher level trim and slight differences in the grilles and taillights.
The standard engine was still the 383 first offered in 1968. The 440 Six Barrel remained an option for 1970. But if you wanted the “King Daddy” under the hood, the 426 HEMI and its 425 Horsepower was your choice!
Only a handful of these Hemi vehicles were ever produced and they are very costly!