I cant help on the stock besides a plain black composite. Knight has them in stock but they are not listed on the website. They are plain but very well made.
As for the LK, my 54cal had a slightly larger bore than my DISC based 54cal.
I would take a guess at trying a tighter fitting sabot and your current bullets. Its the least expensive starting point. You could also try some of the cheaper Rem 300gr 458s in a Harvester Crushib for .452 bullets. That combo usually works well in larger bores.
BTW Knight still offers the 250gr RedHot, its the same bullet as a Barnes Expander MZ but it may use a different sabot.
These come with the EZ load 3P sabot so you may need to swap them or buy the Barnes branded version.
http://www.knightrifles.com/red-hot-bullets-M900590/