67mm turbo....
Or larger... 500RWHP on a .63 A/R should be cake. It is possible to make 500RWHP w/o meth on 93 octane, and very doable with that turbo. Nice powerband too.
Look into what the MKIV guys are running (single kits), and you'll get all the answers you need, to include the A/R, timing maps, fuel maps, injector sizing, use a dual walbro hanger, etc...
8.3:1 seems a bit low IMO. Even if you're going for 30PSI on meth. That headgasket will weigh two pounds**lol. Unless you changed the pistons, but this is on the *cheap.