The "X" member is what gives these cars more frame strength. Most of these frames are not boxed unless a real monster of a motor and a drag race rear end setup is used. But, be sure all the rivets and bolts are re-used. Many people cut into the X member for tranny clearance or mounting and destroy the boxing effect that the stock rivets and plates give. Then you get strange noises, doors act funny, hoods get crazy fit ( not that they fit good to start with).