I thought the 5.0 put out mid 200's stock? Mechanic friend of my brother said it was about 350 hp and it's a race car that's street legal. It has two racing seats and a dash with a tach. That's all the interior. It also has only a 5 gal fuel cell. It's got to be light as hell and when he took me for a ride I thought it was fast as hell. And this is coming from a guy (me) that owns a 9 sec crotch rocket.
Do you own a foxbody mustang? pics if you do.
1986 was the first of the "HO" 5 liters rated over 200 hp. Though the rating was for 225, that was from cherry picked engine(s), IIRC. There was a lot of variance, typically towards 200 hp and slightly under. You could get one that just tore up the pavement, but typically didn't. Mine ran a 15 second quarter mile off the showroom floor at what I consider completely stock (K&N filter, underdrive pulleys, headers), while other brand new GT were turning a more typical 15.5-15.7. This is at a mile of altitude, so not quite as horrible as it sounds.
I have a SN-95 (though I've wrenched on every Ford V8 since the 260). Mine is about 600 pounds heavier in return for not flexing like a wet noodle, having 5 bolt wheels and brakes good enough not to need the "For Mustang Use Only" emergency stop lane at the drag strip. That, and I can actually turn as well as go in a straight line!
Anyhoo, 300 is eminently practical out of a naturally aspirated 5 liter. With matched, good quality bolt ons and a bit of tweaking by a mechanic who knows what he or she is doing. 400hp is a $10,000+ build-up by a pro. Think of as a 4 ghz overclock on a 920 as opposed to a 5 ghz. The first is possible and common, the second is going to be very rare and unlikely for everyday use.