The "modular" 4.6L V8 engine used in 2005 Mustang GTs debuted in 1991 and was designed as a replacement for Ford's aging "pushrod" Windsor engines. These engines have become legends in their own time for their ability to withstand massive amounts of horsepower with very few or no internal alterations. The '05 GT's 4.6L isn't the strongest version of this motor ever produced, but it benefits from the raw strength of its basic design.
Ford took many of the lessons it had learned from its racing experience, in-house R&D and corporate acquisitions to produce an engine that would break the stigma aluminum engine blocks had acquired in the previous decades. Ford deliberately overengineered the Mod motor to withstand far more stress than it was ever likely to experience; the forged-crankshaft motors used in 1990's Cobras are well documented to withstand more than 600 sustained horsepower and 800 in short bursts. The 2005 GT uses a stock crankshaft, but the block's integrity is unchanged.
The 2005 Mod motor is limited by its connecting rods, which can withstand only about 6,500 RPM (perhaps 7,000 for short periods). If you're planning to make big power on a stock block, you're better off installing a bolt-on supercharger or turbo system than trying to rev the thing to death. Boost pressure puts more pressure on the pistons but puts far less pressure on the connecting rods. Ford actually engineered the engine to save would-be hot-rodders from themselves; while the heads are more than capable of carrying increased RPM, the valve springs will lose their ability to keep up at around 7,000. A good rule of thumb is that if you have to replace the valve springs, then you'd be wise to do the connecting rods as well. Get the best you can afford, and don't worry about weight; you'll be happier with forged rods that don't explode than aluminum ones that do. About 400 to 450 horsepower is generally acknowledged as the upper limit for a boosted setup on a completely stock motor. You'll need lower-compression pistons to get above that, anyway.
If you're running more than eight to 10 pounds of boost, the stock pistons will give out before the rods will. Aside from any inherent weakness, the pistons' compression ratio (9.7:1) is too high for the extremely high boost levels needed to get more than 400 to 450 horsepower, depending on what octane gas you're using. Engine knock (detonation) is very dangerous for Mod motors, so tread carefully.
If you're looking at anything more than 500 horsepower, you should seriously consider stepping up to a forged crankshaft. Stock cast crankshafts have been known to withstand as much as 650, but any minor manufacturing defect in the casting will cast the harsh light of reality on your crank's theoretical strength.
