I have a '50 3600 with bed sills from Mar-K.
The second cross sill is the second one from the cab and the end brackets bolt to the bed sides in addition to the angle brackets attached to the bed sides.
I don't have a decent picture of the cross sill installed but my assembly directions from Mar-K I have marked up step 15 - "second and third cross sills ... 26-3/8" and 44-1/4 (was 35-3/16) from the front bed panel....The fourth and fifth cross sills ...63" and 74-1/4" from the front panel (not the 53-1/2" in the instructions)."
