considering 4 speed swap and have questions

I thought the 904 and the 833 have the same length driveshafts, but the front yokes are different. They make adapter joints, but in my experience they don't suffer 400 hp for long. The Dart shaft can be shortened.
The 360 flywheel can be rebalanced for the 340.
Zero balance flywheels shouldn't be hard to find, but new ones are pricey. For a streeter with street friendly rear gears,stick to steel or iron, especially with the 340.The extra weight will help get it out of the hole/off the line.