Larger isn't always better. Most people think it's better. Unless your Titan V8 has a supercharger or turbocharger with it, the amount of exhaust gas flow isn't going to change much out of the engine. Dumping the restrictive factory mufflers will allow the exhaust system to move more gas out of the factory setup.
If you have more exhaust gas coming out due to you boosting your engine, then you can take advantage of the higher flow volume of larger pipes, provided that you can provide smooth bends due to the chassis needs. Maintaining a consistent exhaust pipe diameter is also a consideration for flow (exhaust gas velocity)