There is a seal where the cast iron pump sits in the case, this generally does not start leaking, it would be from the start if it leaks. Also there is seals or metal washers with a sort of teflon coating on them on each bolt the holds the pump in. One could be loose? The other problem could be a cracked case. The center support for the internals on a 350 have cogs or notches that match the case. If there is alot of load in that area it can crack the case.
A cracked case is most common in a 4X4 when the slam them in forward and reverse alot.
I would not suggest using silicon on any tranny, if the any little chunk gets in the valve body, which controls shifting your done.
What I do is make sure the pan surface is flat, use gorilla snot ( wheather strip adheasive) clean both surfaces. Use the gorilla snot on the pan and adhere the gasket to the pan. This way the cork gasket will not push out and create leaks. Don't overtighten the bolts.
Good luck,
Brent
