How much does the 99-04 V6 mustangs have with a 3.8L turbo installed?

There's no single answer to how much horsepower a 99-04 Mustang V6 with a 3.8L turbo makes. The horsepower output depends entirely on the specifics of the turbo kit used (size of the turbocharger, supporting modifications, tuning, etc.) and the condition of the engine.

A basic turbo kit might yield somewhere in the 250-350 horsepower range, but with significant upgrades (intercooler, upgraded fuel system, exhaust, engine internals, professional tuning), you could easily see 400+ horsepower or even more. Conversely, a poorly installed or tuned kit could result in lower horsepower or even engine damage.

Therefore, you need to specify the *exact* turbo kit and modifications to get a more precise estimate. Looking at forums and online communities dedicated to modified Mustangs will give you a better idea of the horsepower numbers achieved with specific setups.