The manual correctly points out that structural rivets are required here, espectially the SSD4XX series to avoid galvonic corrosion from disimilar metals. The manual calls for SSD46-SSBS rivets for the firewall to cabin frame and SSD44-SSBS for the bottom brace to firewall. If you look at the datasheet for SSD46 it clearly states that the rivet grip is .313 to .375! This is not even close to the correct thickness. The firewall thickness is 0.04". The thickness of the 1/8" frame is 0.125", a total of 0.165". In the areas that have the additional 0.063 plate then the thickness is 0.228". The SSD44-SSBS are clearly the correct choice for the firewall to cabin frame since they have a grip length of 0.188 to 0.25, which is much closer. Fire the lower brace to firewall, where the thickness is 0.1025", the SSD42-SSBS rivets were used since they have a grip length of 0.063 to 0.125". Judging by the look of the rivets it was the right decision.
An SSD44-SSBS rivet (Center) penetrating 0.228" upper firewall gusset
SSD44-SSBS rivets penetrating 0.165" at bottom of cabin frame
SSD42-SSBS rivets on 1/16" L-brace at bottom of firewall
