I am going out on a limb here. While the workmanship on that frame looks nice, it is not a good frame. It lacks the proper bracing. It will not have any longitudinal, lateral or axial stiffness because it does not have a K member or any corner gusseting. It does not have any means of keeping the rails in the proper relatioship to each other. A search of tech forums here and on other forums will give you a better undersyanding of what I mean.
Good luck with your project.