Transmission build up

[jking18]

First off, this isn't about the 3 speed in my 89. I picked up a 3400 a while ago and im putting it in a 1996 Beretta that im buying off of a friend. I would like to know how I can strengthen it to be able to handle over 350 hp. I wont be putting those numbers down just yet, I just want it to be able to handle it so there is room for improvement. Just to clarify, its a 4T60e. Hopefully 3x00modified post soon.