So I know its kind on odd question, but I have contacted Ruger with no luck, and finally the maker of the upper on whether it can and should be done. So is anyone running a .458 upper on the ruger SR 5.56 Takedown?
The SR series piston guns are a bit different from standard ARs. Ruger won't respond for liability reasons. Theoretically you could probably do it, but I think the cost will be pretty high.
You might be able to canabalize one of the 300 BlackOut kits to get the parts. But I'm not sure how you would deal with the buffer. And, the takedowns are fine with the open sights, because all of the adjustment is done on the front sight. But if you want to run an optic with any magnification things don't quite work as well, because while the barrels reattach pretty close to where they were the last time, they aren't perfect.
Option two, would be to build someone else's piston upper, and use it on the lower. The plus side to this is the the optic for the big bore, would stay accurate. And realistically the cost would be pretty close to the same.