thanks for the tips guys,
In capping the 150x50x6mm rhs as well as the ends of the 100dia x 4mm tube with 5mm plate in both instances I would have thought a total cross section of 10mm with a 12mm fillet weld all round would have dissipated the stress at these joints sufficiently.
Will post a couple more pics.. the down tube from the base of the A pillar to the suspension uprights has been increased from 25x25x1.6 to 50x50x2.5 in RHS and the effective span shortened by 800mm through connecting a footwell cage with cross bracing to the main chassis rail. This mod increased the stiffness in that section of the chassis by 300% above standard.
Will continue until I get it right I guess..
lol