Yes, it seems that now Great Planes adds tristock at the left and right sides behind the firewall. I was planning to power my SS with a new FS-110alpha, yet the extremely high cost of glow fuel here in Greece changed my mind and I'm now waiting for a RCGF 20cc engine to come. The other owner of a SS with a Zenoah and his reports were also a reason. What I did to reinforce the firewall was to somehow "bolt" it to the structure behind it. Below are 3 pictures, the first one showing the 2 holes opened on the firewall, the hatch to be able to work behind and the 2 "bolts" (notice their shape), the second one shows the back side of the firewall with the reinforcement installed (and you can see the tristock at the sides) and the third the final front side. I still do not know if I'll have to trim the heads that protrude, but I guess that with a gas engine on stnad offs that will not be necessary.
As you can see at the first picture, the sides structure does not fully contact the firewall, and that is because the inside lamination of the firewall is cut slightly bigger. So the sides only contact the firewall at half its thickness. Do you think that epoxying a stripe of fiberglass around the firewall would be necessary? Weigth is an issue with a gas engine in front, though I guess total weight will not exceed the one of the kit.