Carburetor. I'm 22 and I don't misspell that one.
As for a 600hp 4.0, it comes down to money. Anybody remember that guy that was going to bolt up the DOHC Cosworth heads to this block? That's where I'd start- if you could work out all the problems with that setup, you'd be on your way.
Now naturally, of you re-engineer the heads, you'll (hopefully) have dealt with that pesky Timing Tensioner issue, and if you can keep the timing chains from slinging apart at upwards of 5k RPMs, again, you're on your way to 600hp.
Personally, I was raised on a dictum of "all throttle, no bottle," so I'd elect to build a higher-compression motor without forced induction or Nitrous.
This would mean sourcing forged pistons, rods, a crank and a high-volume oil pump.
I've heard that the stock length on the rods in a 4.0 are nearly identical to that of the venerable Chevy 350- if this is true, I don't need to explain the possibilities.
Beyond that, we all know the rest- headers, full exhaust, radical cams, completely new computer tune, hot ignition, E-fan, etc. etc.
Don't forget that nothing in your drivetrain will last more than a week of putting up with that kind of power, and that you'll be rebuilding the motor every 25k miles or so.
hopefully by now you've realized that it doesn't much matter what you do- this is going to cost way, WAY more than it's worth.
aspire to something modest: 275 to 300 ponies out of that 4.0... it's going to crap out at around 100k miles, anyway.
If you're serious about keeping up with the joneses, swap up to a force-fed 347, it's the only way.
~Ken