Lighter and has a taller block but those blocks are only rated to 400bhp so Martin was running over the limit anyway and Millington blocks aparently have a habit of cracking in na form.
Gutted for you Rod however it seems like its more bad luck than anything.
Hope you dont give up as the car has been a big influence to a lot of people and would be a true loss to the scene despite all the banter on here I properly respect your car.