Hate to burst your bubble, but while you can make a slant LOUDER, you're never going to make it sound like a 440. Or even a 273. Ever heard an import with a fart can? Inline fours, inline sixes, V-10's, etc, all have their own distinct sound. The only time you might fool somebody is at idle. Once the engine spins up a few hundred RPMs, the cat's outta the bag.
That said, a slant six can make a REALLY nice sound with the right exhaust...