Will a 4.6L engine from 99-04 mustang fit in 3.8L v6?

No, a 4.6L V8 from a 99-04 Mustang will not fit directly into a 3.8L V6 Mustang's engine bay without significant modifications. While both are Mustangs, the engine bay is designed around the smaller 3.8L V6 and is not large enough to accommodate the larger 4.6L V8 without major changes to the chassis, including potentially:

* Firewall modifications: The 4.6L is physically larger and likely won't clear the firewall.

* Subframe modifications: The engine mounts and subframe will need significant alterations.

* Exhaust system replacement: The exhaust system will need to be completely replaced.

* Wiring harness changes: The wiring harness from the 4.6L will need to be adapted or replaced to work with the 3.8L's system.

In short, while it might *seem* like a simple swap due to shared platform, it's a substantial undertaking requiring advanced mechanical skills and significant fabrication work. It's far more complex than simply bolting in a new engine.