800 horse on "street" tires aint gonna cut it. You'll need Yokohama Auto X tires, BIG letter bias Goodyears (thats what I run), Eagle F1 or similiar. OK call 'em "street" but they better be awesome.
With 800 horse I'd lean toward a straight axle all right, a Ford 9" to be specific. But the ride is SO much better with IRS, I can understand wanting your cake and eating it too.
Rear gear ratios:
With that kind of horse I would go with a HIGH rear gear, like the typical 3:31 ratio common to Jag IRS units. It's just much easier to "control" horse power with a higher gear set AND you GOT to have great traction as well! But with the high rear gear NOW you got a problem with that 5th or 6th gear Overdrive. Shooting for 800 horse means you gonna have a NASTY cam and turning 6500 rpm or so. The motor is NOT going to like highway "cruising" at the lower Overdrive rpm range (unless your hauling ass).
Yeah, you got to work out the details in a number of areas. The cam selection is CRITICAL to overall drivability. The rear gear in large part should "drive" the cam selection. The BIG horse power will soon enough FORCE you to seriously consider the right tires. The trans gearing needs to be worked out. What you DON'T want is 1st gear so freakin' low it's a joke and unusable. That happens a LOT.