On my -6A...
...I installed the Plug as shown on the FWF plans...except I needed to us the next size larger plug.
With the "as called out" hole drilled in the firewall, and the engine mount in place, and the nose gear leg installed, the called out bolt direction (insert from cockpit side of firewall) would not quite fit.
The problem (for me) was that although the bolt shaft would just clear the SS firewall material, the bolt head could not make it through. And if I turned the bolt around and tried to put the nut/washer next to the firewall, there was not enough clearance between the gear leg weldment and firewall to allow the nut to fit.
Also, it just seemed that if I ever did need to remove the bolt, it would be much easier if the bolt were installed in Van's recommended direction, as I could "tap" it out (through the removed hole plug). Not as easy to do from the cockpit side of things going the other way.
When I first started looking for the called out SS plug, I could not find the one that was in the build kit of parts. I did find a non-SS plug, but not the SS plug (check with a magnet, the SS part should not be attracted, a ferrous compound part will be attracted). An email off to Van's was replied with ..."well, we never had anyone actually check, but you are right, they are not SS. We will just need to change our description of the part. What you have is correct."
So, off to the hardware store I went, in search of a SS plug to go with my SS firewall. I did not find any, ended up with a chromed steel plug (next size larger) and after attaching the nose gear + bolt / nut, I prosealed the plug in place.