My vote would be for the headlights with the turn signals built in.
The '40 GMC i'm parting out has the big round deals mounted to the fender, but it looks like they at one point had cab marker lights on there.
My second choice would be to fender mount some cab marker lights.. but if you want to keep from putting extra holes in the truck, those Chev of the 40's lights would be the way to go.
