I am about to install two Guide back-up lights on my 1950 Styline Deluxe. I purchased an after market accessories Installation Manual which has the dimensions for locating the drill holes, however, the reproduction manual is of such poor quality that the diagram and illustrations are all but unreadable. If one of our members has an original installation sheet that can be scanned or reproduced and sent to me, it would greatly enhance my level of confidence prior to drilling holes in the back of my car.
Bob,
This is for 1951 but I'm guessing that it is the same for 1950 as well.
https://chevy.oldcarmanualproject.com/accessories/1951am/index.htm
Go to page 48 and the directions fo backup lights starts including diagrams, and directions for drilling. I hope this helps.
Jeff