I used the V6 flywheel behind my 5.9, just had it rebalanced to 5.9 specs; but that will only work for you if you don't need the crank sensor (not running EFI). This was in a Dakota. It has survived just fine for over 3 years of indiscriminate thrashing.
As far as "sandwiching" a flexplate in, that doesn't work without a hefty amount of machine work to the flywheel, and is only useful if you need the tabs for the crank sensor. Otherwise spend the machining money having the flywheel rebalanced.
If you plan on throwing this in an A body, just be aware of the amount of torsion bar/transmission crossmember cutting and floorpan work you'll need to do- not just relocating the mount to the rear by 2 1/2 inches, but removing a huge amount of the structural upper center of the crossmember, which will all need to be replaced. You need to remove enough to not only clear the body of the transmission, but a bunch more on the driver's side since the dimensions of the AX15 puts the speedometer drive right in the middle of the crossmember and you need to take enough out to stand a chance of getting a speedo cable hooked up (even if you use a 90* adapter, it will need more taken out). All of which needs to be reconstructed to maintain the strength needed for the rear torsion bar anchors in the crossmember, and the tinwork to cover it all back up. I know it's been done, just be aware of the amount of work it will take to do it, and do it right.