5.9 Magnum out of manual truck going to work?

^^THIS^^ LA engines have weighted torque converters to fully balance what is essentially an unbalanced engine, like a 360LA

Magnums are different. They use neutral balance converters, with weighted flexplates.

You cannot use an LA weighted converter because a 360LA is weighted different than a 5.9magnum, and so on.
Bingo! What he said ^^^^^^^^

Justin, the flex plate allows the use of a neutral or zero balance converter. The flex plate has a scallop cut out of it. It only bolt on one way to the crank and the converter only bolts on one way to the flex plate.

EZ peazy nice and easy.