Pre-magnum flexplate on a 46RH?

Factory LA flexplates are non-weighted. Assuming (!) they want you to use the LA 360 balancing (the 5.9 Magnum is different), you would need an aftermarket flexplate like the B&M 10239. If you need the Magnum balance, use the 5.9 Magnum flexplate- one hole needs to be elongated, otherwise it'll bolt up. Wait for Johnny Mac to tell you which one you need, otherwise call them.

"Maybe the better question is, would the A518 flexplate and torque converter work on the BPE 408?"
That depends on whether the A518 flexplate was on a 5.2 or 5.9.