I just asked in the race section not long ago about a 360LA/904.
The 360 I asked about will be internally balanced.
In all of the questions and answers, phone calls made to manufacturers, I found out the 360 (internally balanced) uses the same flex plate as a big block, and either the 727 or 904.
I know you asked about a 360 magnum, (externally balanced I assume) I can’t answer if there’s any differences.
Doubt it helps, but it’s what I know.

Wally.