IMHO, three inch spacing of 1/4” locking mandrel, non-structural/pop rivets to attach a locost floor skin is acceptable.
While the sheets contribute to the spaceframe, they are not considered primary structure. The worst the floor must deal with is the occupants standing in front of the seat when entering/exiting.
Pop rivets are not considered structural and rivet strength varies greatly.
Every rule that I have seen applies to structural rivets for primary structure use.
Maximum spacing can be greater than 12 rivet diameters and the rivet od does not need to be at least three times the thickest panel gauge.
If you find the panels are loose/rattling due to loosened rivets, you can repop with better rivets and/or larger od and/or same size but extra rivets around where the load is greatest. Bonding is another option but it would make future rework a lot more difficult.
A small step panel and heal board at the pedals can prevent floor skin stretching and rivet loosening/shearing issues in the future.
If building monocoque, follow the rules for primary structure.
Potential issue to look out for with a pneumatic/hydraulic rivet gun:
https://www.doityourself.com/forum/hard ... plode.html
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