Kickdown not attached 904 won't shift gears

Yes, it's toast. You MUST have that linkage hooked up. The correct name for it is not kick down linkage, but throttle pressure linkage. As the throttle is opened, the linkage is pushed back on the lever on the transmission valve body and increases the line pressure in the transmission. As the line pressure increases, the clutch pack "holding power" also increases to match the rise in power from opening the throttle. Without the linkage hooked up, the clutch packs have minimal pressure holding them together and the clutches will slip. Since high gear puts the most leverage on the drive train, it will burn up first, followed by second and then first. If your transmission is slipping in any gear, the damage is already done.