If you have a piece of the female spline just drill a hole in the end of the block and silver solder or braze the sleeve in the hole. Even if you do not have a femail piece of the spline it would be easier to make a short section by hand using files and than solder it in. It would make a difference if I knew what this was going to be used for. If its a slow turning machine you could possibly get by with drilling holes through your 6 in piece of stock and putting in stubby pins that would ingage the male part of the spline. Good luck and think simple...