Your 765 should be a straight shot as well, if not, its probably misaligned.
Misalignment of the stacks on the scale berks is easy to do if you've taken it apart for anything, some were even delivered from Lionel new that way.
You'll have to remove the four screws underneath, two up front recessed into the underside of the steam cylinders and the two which are the second screw in from the the rear of the cab.
Lift the shell up a bit and try lining up the stack with the opening first,then lower the cab down.