A couple of more thoughts.
1. If the cab is still attached to the frame, it can affect the frame. You may want to unbolt it and support it so the frame is free to act on its own.
2. This may or not be complicated, but I will try to explain. If both rails are level and straight (the majority of the TF frame is straight) then any place along the frame that is checked across the frame rails will also be level. At the front where the frame drops down, this should also be level if the frame has not been damaged. This drop can be checked by referring to the assembly manual. The assembly manual will also give other references as to the frame being within specs. Something to remember is that the flanges on the frame are not always square to the web of the frame. The level may be sitting on the toe of one side and the heel of the other side. This can cause out of level conditions from side to side; while the rails are level to each other.
3. Another thing, does your level read true. That is, does it read the same when it is rotated end for end with the same side up.
4. If the frame is what it is, 1/8 higher on one side. Remember that the difference can be split and that makes one side down almost 1/16 of an inch while the other side is up almost 1/16 of an inch. When it is looked at in this prospective, it may well be within the tolerences needed for your cross member install.
Good luck on your install, and remember measure, measure, check and measure and check some more before welding.
Have fun.
Lonnie
