5.9 Magnum flywheel - DIY with bolt-on weight??

Well according to this book,
Chrysler Performance Upgrades
by Frank Adkins
published by CarTech as part number SA-60 in 1999.
ISBN 1-884-089-40-2
The Magnum 360 flywheel counterweight is is 14 oz-inches, which makes your maths pretty much spot on.