Seacock backing plate yes/no?

[Jado27]

1973 Catalina 27. I'm changing out the original through hull and gate valve for the sewage. (Has been corroded shut for years).

I'm putting in a real brass seacock with flange and need to bolt the seacock to the hull. Hull is 7/8inch thick where I drilled. Do I I need to make a backing plate for seacock or just go ahead with bolting in seacock then screwing in through hull. Seacock/through hull size is 1.5 inch.

Thoughts? I don't want to haggle with fiberglassing a backplate in if hull is thick enough.