I've stitch welded the join between the chassis legs and inner wings. I've also welded around the front gearbox mount area. The most important bit though is the lower bulkhead to chassis rail joints, a notoriously weak spot that is prone to cracks and splits particularly on mk2f models. I've formed diagonal gussets here from 2mm thick sheet and welded them in.
Sorry, I'm no good with pics, but I hope this helps.