Ironically thats one of the most dangerous setups for an escort cosworth. The standard injectors cannot support a stage 1 setup using a 0.63 T34 as they run very lean at high rpm if running anything over 12psi boost.

In this case I would suspect lean running on boost to be the culprit rather than exhaust back pressure. (As it is again excessive heat that causes the wastegate area to crack)

Solution is to run bigger injectors and have it mapped properly.