I have the bolt and retainer that you need. I would be glad to ship it to you if are coming to the Slant 6 Spring Nationals in April 2018 at Mo-Kan Dragway.
If you are not going to make the race, I'm not sure I have the bolt.
The factory bolt is grade 8 and sure is nice.
We have used a 5/16 non step grade 8 bolt on the Lancer for about 15 years with increased valve spring pressure over stock with no problems.
I will get bolt mailed to you within a couple of days. Sure would like to see your wagon.
Mark