Oh mate don't be mistaken, this isn't your fault
CRD have done every thing to this car and the 3.5" dump and good cat should have been done or at very very least talked to you about when the 6262 was fitted
Your setup is what i would consider the perfect setup for a R34 GTR, even better if RB26 block based for fitment, and what the R34 GTR should have had from factory or at very least a 3.0 with twin -5s