On the one hand, I'm pretty sure you could just run it straight. People do that in the stick world all the time - using 7.3 flywheels on a 6.9. Obviously, proper balance is best, but I don't think the balance is far enough off to make a difference.
I also just did the opposite on my '87 - used a 6.9 c6 flexplate/flywheel on the motor in it, which ended up being a 7.3(what I pulled out of it when I got it looked the same, so it'd probably been running with a 6.9 flywheel/flexplate for years).