'65 Barracuda 318 LA Transmission Questions

400 from 318cubes, for the street; is IMO crazy .( It's even hard for a 360.) You're looking at 1.26 hp per cube, so she will have to be a high-rpm engine to get there. That means you'll need to carefully co-ordinate all your parts,specialized machining, and it'll probably be cheaper to use aftermarket ported heads, than to use the factory LA stuff. Headers will be mandatory, as well as a big cam with lots of overlap.
The thing is; at 3000pounds, you only need 300 to have fun with. And that is .94 hp per cube and is much easier and cheaper to do. Any Mopar trans can handle that.
A basic 360 at .94hp per cube is ~340 hp and makes a killer early-A.