Around here a stomp starter IS a theft deterrent. Every show or cruise-in I go to at least one person asks what the extra pedal on the floor is.
X 2!
The stomp starter is likely the best theft control feature on these trucks.
Locking the hood can help slow a would-be-thief down.
I'm sure locking driver and passenger doors could be had if you swap the doors from a 1950-1955.1 truck. You would also get wing windows.
The fact of the matter is these trucks can be taken if desired. Best defence is to insure them properly.
My 59 gmc isn't a pretty truck. It is freequently left parked in San Francisco with both dors unlocked and the windows rolled down... even overnight. Paint a couple of body panels primer color and nobody will want it!