It sounds like you're basing this off of word of mouth numbers, not dyno results and don't truly realize what 700hp entails.
My father had a 70s F-100 with a balanced and blue printed 428 under the hood. It dyno'd out right around 600 naturally aspirated and would lift the front tires up leaving the line. He drilled the shocks out in an attempt to keep them on the ground. 4mpg highway. Two 3/8" fuel lines. He broke transmission supports and motor mounts loose to where the transmission would hit the cab of the truck leaving the line. I highly doubt your uncle's Saleen is pushing that kind of N/A power because that's what it takes, and it's hardly streetable (or possible with a 4.6 3v N/A).
Now, if your uncle's Saleen really is pushing that kind of power, it's got a blower, which a Saleen blower won't reach those levels on a 4.6, so he's got larger blower. You say it had 425 horses stock, which a Saleen S281SC had 465 at the crank stock (around 400-420 at the wheels). The Saleen blower is simply not big enough to feed enough air for 625 at the wheels (maybe 625 at the crank with supporting mods, but I doubt it), hence my reasoning for pointing you towards GT500 blowers.
Now, I'm not trying to rag you on any of this, only offer some advice. If I were you, I would buy the stock GT and drive it. When you want more power, go with the DOB kit: M122, 2010+ CAI, 2013 HE & IP, FRPP 52lb/hr injectors, dual GT500 fuel pumps, but don't touch the motor. Dyno it at 450rwhp and leave it for a while. You sound like you're stuck in a peak numbers game and don't realize this car will be under 3800 pounds with this set up and be producing slightly more power than a stock 2007-2009 GT500 (and drive like one, daily driver material). Once you want more, build the bottom end and crank up the boost to hit that 575 mark. Unless you're devout on the drag strip, I doubt you'll want to reach higher than that, but if you do, that's where the real money hits and it gets serious.
Regardless of what you choose to do, I look forward to seeing your build.
2007 Mustang GT 5-speed 3.31 gears, Hidden Hitch Receiver, Barton Short Throw, JLT Series 3 CAI, Steeda UDPs, Off-Road H pipe, Borla Touring Axle Back, and Brenspeed SCT X4. 255/45ZR18 front 295/40ZR18 rear
Piecing together DOB:
2011 GT500 M122, TB, HE, & CAI | Bosch IWP | DW 340lph pump | FRPP 55lb | AEM wideband