300HP on a 3.8L V6? You will need a bult bottem-end, super- or turbocharger. I think the V8 swap is wiser in that regard.
I am with Fordman, I think if you are dead set on a V8, get a 5.0L Pushrod engine, HCI, and you will have 300+ for a pretty good price. Along with everything else (suspension, computer, tranny, rear-end), it will be more cost-effective than an equivalent 4.6L swap (not sure how Emissions testing will be affected either). I am pretty sure if you put an older engine into a newer car it will not pass but I'm not totally clear on that.
Matthew 26:52-54 / Go Pack Go!
1987 Mustang GT Hatchback - 2V Mod Swap In The Works
1997 Lincoln Town Car, PI Intake/Cams, Sofa-On-Wheels, "Fat Tony" we call it
2001 GT Convertible - D1SC / Geared / Cammed / 440wh 400wt - TOTALED 2/2019