Are the tailgate chains on an M715 supposed to be long enough to support the tailgate in a flat position (level with the bed floor) or are they just there to keep the hook from getting lost?

I have no chains on mine and I am going to go to Lowes and buy the stuff to make some. I am not interested in originality, just function. Any idea how long these are supposed to be? Also what size (diameter) chain should be used?

Thanks.

Bob