I'm afraid you didn't do yourself any favors by using a gasket material thicker than the original. Gasket needs to be thin - a thicker one will just reduce the pumping ability of the impeller. It needs a close seal to the front plate with very little gap between plate and impeller. None if rubber type - slight if metal. Most are paper thin and that is all that is needed and is use as a binder and vehicle for your
sealant of choice. The gasket merely keeps you from squeezing out all the
sealant and compresses into any pores of the metal mating surfaces.
For self made gaskets - Put a well fitting gasket on your
photo scanner and print one out at original size and cut it out with scissors. I use a hole punch like for leather belts for the screw holes. That's what I do for my
Jabsco rubber impeller pumps. Aviation Permatex works well as a sealer. Right Stuff Blsck is good too.