A unibody or "monocoque" frame uses the structure of the car's body to maintain its shape. Unibodies can be strengthened with either add-on chassis reinforcement or spot welding of weak spots in the structure.
Most full-frame vehicles use a perimeter frame arrangement. Though it is possible to install heavier frames with wider rails, the cheaper and easier solution is to modify the existing one.
To resist the twisting force exerted by a high-torque engine, many builders use an X-brace to tire the frame together. This large cross-member uses two diagonal braces that connect to the frame just behind the front wheels and ahead of the rear.
Boxing the Framerails
Most perimeter frame vehicles have one or more sections that are open on the inside. Filling in the open side of these sections with sheet metal to "box" them in dramatically increases strength.
Some choose to use a heavier-duty frame from a larger vehicle, cutting it down to fit their own. This option is viable, but many consider lightening the frame in non-crucial areas.
Increasing an engine's horsepower puts a strain on all components of a car. Making sure your frame is up to the task is possibly the single most important thing when drastically increasing output or swapping engines, because it's the one thing that everything else depends on to work.