If you're going with Reeder and you have and like the 500 S&W, why not just skip the used gun at your shop and go straight to the goods with Reeder's stretch-frame single action in 500 S&W Magnum?
He even calls it the Howdah.
It's only $2500, which after buying the used gun and paying for a custom build on it, the stretch-frame gun doesn't cost much more.