rivnuts or not?

[SteveH]

ive been trial fitting my seats recently and have decided im going to mount them on 25mm x 25mm tube thats welded to both chassis rails
my question is what would be the best or preferred method of fixing the seat runners to the tubes
do i drill straight through the tube and put crush tubes in and bolt all the way through the floor
or do i use m8 rivnuts and bolt them up that way
using the crush tubes would make it very solid indeed but does the roadster need to be like that