https://www.amazon.com/gp/product/B007DKQBXA/
https://www.autozone.com/gaskets/exhaust-flange-gasket/p/mahle-original-exhaust-flange-gasket-f7139/146118_0_0
Therse fit my 78 k20 350. I have this style manifolds
Inside measurement is 2.5 inches. Worked for me. Good luck on the part search