installing mustang fuel tank & flipping the rear shackle

[nikwho]

Ok, I bought a '69 18 gallon mustang fuel tank. it's too wide to fit between the frame rails but not by so much that I can not make it fit with a little trimming of the inside of the frame rails. I took out the large cross member that went side-to-side between the two shackles to make room for the tank. It was molested pretty badly so I trashed it and will make a new cross member. There is a ton of flex in those frame rails without that cross member there. Have you guys been happy with simply installing a new cross member at the very rear of frame behind the fuel tank? It seems that it will still flex a lot with the movement of the suspension. Also, I intend to flip the rear shackle hanger. I have also flipped the rear axle and removed two or three of the eight or nine leafs out of the 3/4 ton leaf pack to soften her up a bit. I'd be interested to hear if guys have had problems with their frame twisting or flexing without the cross member going straight between the two shackle hangers.
Thanks, Nik