Bill,
I can’t help with locating the support. Classic Roadsters running boards attach to the front and rear fenders and to the body, but do not have an additional support.
But regardless of where you locate that support, I recommend you not let anyone actually step on the running board. I’ve never tried, and have never seen any of my TD friends try.
Just my humble opinion.
Paul Mossberg
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)