[63gmc4x4]

you must have a different 401M if the port size is the same as the 305, my 351E port size is alot bigger thus using a smaller 305 gaskets it hangs into the flow of the air/fuel mixture coming in, and or doesn't cover the complete gasketa surface area onto the outer edge ,sure it may seal up okay but may leak over time I just dont like the gasket haanging in the way from intake into cylinder head, did you see my pictures?