Does anyone know if Blitz do a back box for the 03 sti type uk and whether it will connect onto the existing centre section. OR if i buy the nur spec r system whether this back box will fit onto existing centre, then i can fit blitz centre section later.
(With it being a brand new car dealer will only let me fit back box because of warranty)

 

If you get the back box only, it will fit as it is designed with a reduced entry to bolt on the 2.5' stock center.

BB and center Blitz do not exist in a "short" version to fit the 4 parts (2 cats turbo back: DP, cat pipe plus center and BB) european WRX systems as far as I know.

If you really want to, you can bolt a classic DP (up to 2000) and then bolt the Classic or japanese Blitz system. I did on my STi03

Note that a classic catted DP will not fit STi without serious mods as M6 gearbox I way much larger than M5 !