Rear Diff question

[STL_66 Chevy]

i am replacing the cover on my rear diff. and i have a couple questions...
1. some people say to use the gasket, some say use silicone! a couple said use both! what method should i use to beat the leaking dif battle?

2. i dont have a drain plug.... how do i know how much oil to put in? do i fill it until it comes out of the fill hole on the top passanger side of the dif housing?

thanks for any help you can give!