Since no one has put up a thread like this, here is one way you can do it.
I modified an existing 7M-GTE downpipe (3" ID)
First I used a 3" v-band clamp and welded it to a 90 degree bend:
(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1604.jpg
(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1607.jpg
(Click to enlarge) http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1613.jpg
Next, cut off the existing 3 bolt flange:
(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1612.jpg
(Not pictured, but to save some space near the firewall, I inserted a 'pie-cut' help tighten up the radius.)
Don't forget to add the o2 sensor flange:
(Click to enlarge) http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1619.jpg
Next, just weld it up to the old downpipe:
You're finished:
Maybe later I'll show you how I converted the SSAC manifold to a TiAL 44mm v-band... but it's basically the same procedure.
I modified an existing 7M-GTE downpipe (3" ID)
First I used a 3" v-band clamp and welded it to a 90 degree bend:

(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1604.jpg

(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1607.jpg

(Click to enlarge) http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1613.jpg
Next, cut off the existing 3 bolt flange:

(Click to enlarge)
http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1612.jpg
(Not pictured, but to save some space near the firewall, I inserted a 'pie-cut' help tighten up the radius.)
Don't forget to add the o2 sensor flange:

(Click to enlarge) http://img.photobucket.com/albums/v635/BrentlyandMeghan/DSCN1619.jpg
Next, just weld it up to the old downpipe:

You're finished:

Maybe later I'll show you how I converted the SSAC manifold to a TiAL 44mm v-band... but it's basically the same procedure.