If you're going to use a posi setup, I'd call Tom's Differentials and talk with them.
I remember that some of the differentials has 8 plates and some had 10. The spider gears were also a weak spot but I believe Tom's makes replacement high strength spider gears. They've been doing these '55-'64 style drop outs for a long time...many of the 409 guys use their stuff. Might be worth the call if you're committed to using a posi. -Al
https://tomsdifferentials.com/toma.html