Originally Posted by nkaminar 
I have the F-100 box on my car. Not sure who did the work as it was on the car when I bought it. The end of a stock box has been welded onto the bottom of the box so that a stock light switch can be used.
That looks like the answer! I should be able to find a junk box to get the original end cap part. I don't want to destroy my original Model A box which is still useable. That way I can do the conversion on another box without tearing down the car until the modified box is ready to be installed. THANK YOU!