I was planning on buying a d series turbo manifold but i got to thinking about just fabbing me a wastgate flange and welding it onto the existing hole where the o2 sits. Would this work? How well do you think the welds will hold?

 

Yeah it will work, Ive seen it done. You gotta make the hole bigger for sure ! Just make sure when welding steel onto cast that you heat the cast up with a torch till the aera you are going to weld is cherry hot, make sure its red,red.. Then put flange on and weld away..
Its good to have the manifold secured in a vice while its red so you dont move it or bump it and or get burned.. If you dont heat up the cast it will crack off and roll down the street under pressure. Yeah it might hold for a month or so, but it will break off if you dont heat the cast good.