I think your on target close enough.
I also have the old 904 / 19 splined converter. I had it re stalled @ Pro Torque, good for 650HP and laughing gas.
There not paid for yet. (Should be soon though....)
But I have bare builder 308 or 596 heads, I forget. In fact, I’ll have to dig them out later.
The Magnum heads are a good choice. A bit more involved that a straight LA head swap.