The type of machining that needs to be done (you do NOT cut the flexplate) is more elaborate than rebalancing the flywheel to 5.9 spec, which there are diagrams for that are easy to follow. I would NOT do the sandwich method unless as a last resort, since it moves the flywheel deeper into the bellhousing and can mess with starter engagement and clutch release clearances.
Do it right or not at all, this is a 50+ lb. weight spinning at 5k rpm or more.
In a pinch, use this:
Bolt on Flywheel Weight for 130t Ext. Balance 360 Magnum Mopar Dodge A833 5.9 | eBay
I have several for different applications (5.9 Mag, 360LA, late 340 ext. balance) that I can slap on the flywheel I use on my run stand so I don't have to chase down a new flywheel with every different build I do... they work fine. Just watch your clearance between the weight and the dust shield.