there is a thread discussing this, i can,t remember which section its in , maybe its in the chassis section, I believe it was posted by Ashg & its info to do with fitting "seat runners" or possibly has " metro seat runners " in its title, sorry i,m being no help but its a while since this was posted.
It also depends on what seats / runners you intend to fit,
I,ve welded in 25mm box tube to fit my runners to, as i needed a slightly higher seating position
cheers
andy