Do you know the year of the frame? To be that far off at the rear, your holes in the frame may not be in the proper location,
If you have a '47 or a '48 frame, someone may have drilled the holes in the earlier frame to accommodate the newer design of shackle style mounts and drilled them too high.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission