Quaife will be fine at 500hp, it will be other things that can fail, at if using a standard input shaft get everything superfinished , don't waste your time shot peening the bloody shaft and the two cogs needed as they were done at the factory! Someone keeps banging on about shot peening but the truth is it's hardley worth it on a 25 year old 100,000 mile gearset. Three things break gearboxes wear,torque and bad drivers.
tbh you are best to get a quaife Mainshaft as that will be new and will take all you can put thru it....after all you have the rest of the gearset, if building a box look at getting the cases cleaned, blasted and cryo treated as that will at least help re stabilise the old cases as they become brittle with age and heat cycles.