1000rwhp...
Is a MkIII really streetable at that point? Driving would probably be a never ending traction competition
A roll cage and body reinforcement, beefy tower bars, extra subframe bracing, beefed up traction bars, and the most massive brakes possible, are what i can think of off the top of my head.
Pulled fenders to fit some wide ass tires in order to put 1000hp on the pavement. Any wheels above ~11.5-12" will require a widebody job (I think I've heard of someone pulling the fenders 1.5", I dunno though). Getting a 365+ tire on a custom rim would probably help traction quite a bit.
Adding a dry sump oiling system and a high volume pump may give more room to reinforce the mains on the crank... I think... Anyway, a custom oil pan with a drain, a remote sump/pump and remote filter, cooler, ect. There's articles about it around SM.
That's a seriously costly project, and I respect and admire someone who'd undertake it. Just remember that rolling into a lump of money is a good opprotunity for some smart investing, ect. Real life has to take prority.... I speak from experience because everytime a fat paycheck rolls in I need to take a bite out of reality in an attempt to not run out and spend it all on the supra
If you're sure about spending the money, go for it... good luck