What mods for my 904 when using the BPE 408?

Assuming the converter is neutral, as most aftermarket ones are, then Yes, you use an LA 360 balanced flexplate.

The Balancer (which we sell for under $90, and include on most dressed engines) uses the magnum weight for our engines)
Mopar Small Block Harmonic Balancer | 273 | 318 | 340 | 360 | 408 |

if its purchases on its own, it comes with various weights for all SB balances.
OK cool, so, IF I don’t have a flexplate, I use a B&M 360 flexplate designer for neutral balance converters.
Correct? And a front harmonic balancer like yours or one for the listed years on the Blue Print site.