Brownell's has 2 lenght cross bolt wise, if you order magnum lenght they're a tad longer than their conventional cross bolt. Try contacting their tech. dept.
The threaded tube is longer but not the screws, you could make a cross bolt tube by taking a blank steel tube and thread each end & cut to desired lenght.
If still you're still left in the dark try Cape Outfitters(U.S.). This outfit has gorgeous stuff in big caliber weapons & accessories.