It is common for this to occur on the front channel holder, the channel on the rear of the door is replaceable but not reproduced as far as I know.
Rear Channels You can cut the rear to fit the front and then you need to spot weld or J-B weld it in place, the front is welded in place from the factory. You can find these channels used and NOS on eBay or you can place an ad in the Swap Meet section, (right or left) will work for the repair. This is not a simple fix the door needs to be stripped and most likely off the truck to work a lasting front replacement.
Inside of Door The 1951-55 version as shown is completely different.
1951-1955 Version
