3.8L 2004 Ford Mustang modifications.?

if you want the performance of a comprable/same gen GT out of a V6 (and you're not sitting in a 300HP 2011+ V6) then you're only logical option is to sell it and get a GT! (NOT KIDDING)

Otherwise your only options would be to have to spend thousands of dollars (if not tens of thousands more than the car is worth) to do something like rebuild of the engine from scratch so it would be able to handle everything that comes with higher performance.

Cause as it is the 3.8L engine is CRAP for performance, putting cheap bolt-ons and exhaust on it would be a waste of time, and if you put a SC on it as it sits it would basically just destroy the engine!

Being totally honest here, it's just a lousy engine for anything more than half-decent MPG anyway you look at it, in fact while there are more powerful V6's out there now, I'm convinced these cars/engines are basically the reason V6 mustangs get so much crap even today!

Plus, I know from personally owning a 2000 V6 that they don't even like speed either....the front end gets unnervingly light at 100MPH (which is NOT supposed to happen with performance cars) and besides that they are limited to 115 because AGAIN if you took it off the engine would basically destroy itself!

Frankly you are just straight out driving the wrong car if you want power out of a mustang....and especially a V6 since the 2011+ ones have even more HP than a 04 GT would.

The other guys have it. I don't see an engine swap mentioned though, I would assume a later/better V8 could be swapped in the car? Let the Ford guys answer that, but NORMALLY if the engine was used in other vehicles the same year of yours (like V8 Mustangs) then they will fit in your car. I understand the sentimentality, finding a wrecked V8 mustang as a donor shouldn't be too hard, but you'll need/want the whole thing as you'll need a TON of parts. Couple thousand should get you the donor IMO.

"What is the best way to give this vehicle GT performance without selling the vehicle? It was an unexpected gift and very sentimental to me. "

You're going to have to spend a grand or two on forged internals (rods, pistons, and crankshaft), and another $4k-5k for a supercharger. That will get you performance that slightly exceeds that of a stock GT. Bolt-ons aren't going to come close to getting you there.