Eric, some things to consider...
1) If building a 500 S&W bolt rifle requires a magnum Mauser action and bolt, then there are many, many better cartridges than the 500 S&W... it's a handgun cartridge, after all.
2) Being a handgun cartridge, you're unlikely to get anything out of that 28" barrel that you wouldn't get out of a carbine. Pistol powders are simply too fast, and the case capacity is too small to use the slow rifle powders that a 28" barrel could optimize.
3) You're looking at a five-figure gun, when everything is included. Custom barrel, custom bolt, magnum Mauser action (these are not cheap), then you're gonna want to put a high-grade stock on it (hell, you're gonna be well over five grand just for the barreled action and the bolt, and you're not going to want to put that into a cheesy plastic stock), the right scope for such a rifle is not going to be cheap either... You'll be well over ten grand by the time you're done. What about a 500 S&W bolt gun would justify that price? (That question has more than one answer, by the way... it's just that you need to ask it of yourself before you commit)
Bottom line, it's not going to be easy, and it's not going to be cheap. Are you sure this is what you want?
--Shannon