I removed the fitted passenger BB Classic seat, now I know the dimensions are good, and welded the 40x20mm bars to the runners. Until then they were bolted together, which seemed solid enough ... but better safe than sorry.
DaveP - hope this help visualise what's needed. You will see below gaffer tape covering the first holes I drilled. This was before I realise the chairs need to mounted offset to the runners. I don't mind about the gaffer tape, as these bars will by carpeted.
