Boucher,
I am not wanting for you to buy that DP, just to try & copy that design to help w/turbo spool up faster & flow better than the stock set-up you are currently using.
Don't need to use stainless, but a low cost alternative to using pricey 304or more $$$ 321 stainless is 409 stainless,or use mild steel as you have already.
I am pretty sure you could just purchase the flange w/wastegate valve from some Turbo Buick forums,just an option to think about.
Have fun doing your project & take your time & do it correct the first time if possible.
MBHD
