Correct, the drill was to unbolt the head, screw on the extension and reinstall the head now out about three more inches. Be advised that the thread on the mirror arms is a non-standard 3/8-20 !
http://67-72chevytrucks.com/vboard/s...d.php?t=564182
My advice is to keep the DG4s unless you need more. The campers are a pain to live with due to their basic irreducable width. DG5 West Coast are just too big for most needs and the standard 71-72 (aka sport mirrors) are about useless. On your 70 the correct base mirror is a reasonable square head on a fixed arm but many people are happier with the earlier round head on the same arm.