Those pics definitely look like the wastegate is fed off of the headers' up pipe below the turbo.
My guess is you are looking for a factory Buick 4-bolt downpipe flange pattern that feeds the wastegate from the existing wastegate hole that's on the turbine housing right?
From memory, the DP301 had the exhaust discharge pipe and the wastegate feed pipe on the same flange and was for use on Buick 3-bolt turbine housings.
I believe South East Turbo is making what you are looking for in 3".
eBay Motors: New Buick Grand National 3" External Gated Downpipe/V (item 120165416611 end time Oct-01-07 13:11:13 PDT)
But I don't currently know of anyone that is making it in 3.5"
If anyone could though, I would have to say it would be Jack Cotton and his crew of fabrication magicians.
Welcome To CottonsPerformance.com!
The problem you run into is the area between the 3.5" hole in relation to how close it would be to the wastegate discharge hole when you put 3.5" tubing beside the 1.5" discharge tubing that will feed the wastegate, it makes it very VERY difficult to weld the tube properly. Nothing like this is ever easy tho. I have just one question and this is something that has bugged me from the beginning with downpipes.
Why is it that people who do not need a 3.5" downpipe always insist on having to have one?
Now, I'm not saying that you don't need one J-B Racing, in all probability, you might in fact need one. It's just that people always get on the kick of bigger is better, when in 9 out of 10 cases those people screaming for 3.5" downpipes have 11 and 12 second cars. I've said it before and I will say it again.
If your car is slower than 10.50s, you do not need a 3.5" downpipe.
I proved this fact myself 2 years ago with my old car. It had a simple 3" external wastegate downpipe like one very similar to SE Turbo and ATRs DP301.
Sorry to rant guys and gals, but this is personal to me.
Good luck with your search.
Patrick