Hi,
Im cruising a jzz30 with original 1jzgte, so you are right with the downpipe, it seperates after the cat, You should build your own downpipe to get a 3" diameter, stock is smaller & restrictive, the catalitic converter is located in the midpipe, i used a hks midpipe to decat. don´t buy the 1jz downpipe from ebay, it won´t fit, it´s made for the JZA70 MKIII Supra.
good luck!
rocka