Really good question! I've wondered that myself so I looked in my 1965 Chevrolet Truck Shop Manual Supplement and on page 1-2 they show the driver's side at 3 O'clock and on page 1-3 they show the driver's side at 6 O'clock and on page 1-4 the passenger side is shown at 9 O'clock, so I say whatever you like.
I think driver's at 3 and passenger's at 9 looks best to me.
That's my two cents.
I noticed that in my 65 manual too! Page 1-2 shows another horizontal handle also.
Here's an old thread I participated in back in Jan. 2011 that shows quite a few original trucks.. it appears the door handle being horizontal was "factory" on 64-66, and vertical was "factory" on 60-63? But my '63 Service manual shows the handle both forward and down too? Perhaps it was "up to the assembly line worker" back in the day?
Some very nice low mile original survivor truck photos in this thread...
http://67-72chevytrucks.com/vboard/showthread.php?t=439322And here are some old advertisements which seem to add to the confusion:
http://i645.photobucket.com/albums/uu176/lakeroadster/Generic/attachment_zps9e749803.jpghttp://i645.photobucket.com/albums/uu176/lakeroadster/Generic/250020vs201500_0001_zps8a390ecb.jpg
