'99 V70R exhaust measurements/exhaust upgrade advice

[bmdubya1198]

I think the stock downpipe "steps down," if you will, from the flange to the pipe, so there is no need to modify the flange. From what I've read, there is absolutely no modification needed to the turbo outlet flange.
Obviously a pre-made DP will be done for you, but for a custom DP you can get the correct flange from someone like CJ Yother https://www.yother.com/product-p/1019-af-ms.htm.