The short shaft will be like a '68 box, the long shaft will be like the '65-'66 box.
The end of the shaft on the '65/'66/early '67 box is where the nut holds the steering wheel on the shaft. The end of the late '67/'68 box is at the rag joint just outside of the box. Search on Ebay for a '65-'66 box and a '68 box for comparison.
The early column will be a long, smooth column to the floor. The late column will be convoluted, sorta like a shock absorber boot.
The late column/box is a safety design, the column collapses in the event of a head-on collision, whereas the early column/box has been known to impale the driver occasionally.