I need advice i owe 2003 ppp scooby, id like to change exhaust on 3" but im thinking only about change mid section without resonator and back box.

id like to fit Invidia N1 which is 3" against prodrive mid centre 2.5"(i think is 2.5")
i stay with up pipe catted/downpipe catted/ invidia mid section without resonator and invidia back box.
how fit 3" mid pipe to down pipe(which one is 2.5" but im not sure)

link to what id like
http://cgi.ebay.com/ebaymotors/Invid...Q5fCarQ5fParts
and below what is fitted on wrx 2003 ppp against standard wrx

For newage UK cars:

WRX - up-pipe catted/ downpipe catted/mid-section catted plus has resonator. None are "high flow" cats.
STi - up-pipe non-catted/ downpipe catted/ mid-section catted plus has resonator. None are "high-flow cats.

When PPP is fitted:

WRX PPP- up-pipe catted/ down-pipe catted/ mid-section de-catted plus has resonator. None are "high-flow" cats.
STi PPP - up-pipe non catted/ down-pipe HIGH FLOW SPORTS CAT/ mid-section de-catted plus has resonator.