I got my 7-way replacement cable from ETRAILER.COM... Seems like it came in only two lengths...
There will probably be two types called 7-Way round pins and 7-Way flat blades
Be sure to get the correct one..
Might check with them..
If I was doing this and come up with 6-inches short I would replace the whole cable with a longer cable and the proper-molded end on it. These 7-way cables all terminate in an electrical junction box on the trailer side and should be a color-for-color wire cable lead replacement routine.
Adding an ADAPTER to me would be introducing another easy failure point. I have enough of those already without adding more...
ETRAILER has a video of this repair routine on-line.
Roy Ken