as posted above no gm part header gasket was ever used on the turbo , the only part gm carried and most stores have on hand that does fit is the foil faced for NA and wont hold up to serious boosting
if you want header gaskets get the felpro 1400, or more $$$ SCE copper #4472 for stock heads
turbo to header flange to header ..none were used on our turbos , riviera had one but no more .
if flanges are flat you shouldnt need one
but if you want
kirbanperformance.com sells one but they wont hold up to serious boost
SCE makes one in copper #4172 , try cottonsperformance.com about $12
havent tried them but rjcracing.com sells milam laminate gaskets for headers and turbo,