Consider using a compound designed to replace gaskets, made by Permatex and called "The Right Stuff". This is not silicone but a rubber type material. It comes in pressure can and also in a tube designed for your household caulking gun. It is a little pricey ($20 for the tube) but you get multiple uses from it. I have used it on the differential cover and rear axle cover on my '50 Chevy COE and on the oil pan on my Diamond T. It is so easy to use with the caulking gun and so far no drips or even seepage. I was told by the truck parts store that made me aware of it, that it is OEM on certain Caterpillar oil pans. The directions say that you can put the vehicle immediately into service after mounting the part.

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix