possible 360 build

I totally get your dilemma.
I too,would totally like to use the flat-tops
if
they come up; close to, or out of, the decks.
But if not then,whatever; and I wouldn't lose sleep about balancing at this point. First I would mock it up and measure the deck-clearance, to see what's what.

Internal balance is pricey, and for a streeter, I don't see an issue with bolt-on balance. The LA slugs are not That much different from the Magnums,so it should not be too difficult to make bolt-on weight work.
And if you have to take some metal off the tops of the LAs to fit into the Magnum, that would be moving in the right direction. So;
First I would mock it up and measure the deck-clearance, to see what's what.