323 355 or 373 gears??

What gear ratio does your dana have now? Most likely 3.54 or 4.10. The only place that I know that has 3.23s is Dr Diff, he had them made.
If you have 3.54, that's an excellent compromise gear, 4.10 is very steep for 80mph, mine is on the high side of 3700 at 75, with 27"tall tires.
Most people who want to move up to a dana don't have one, so the investment is a little daunting if you already have an 8 3/4. Since you have a 7 1/4 and a dana, the dana is a no-brainer. The dana will be fine for a blown or nitroused 408, no problem. A nine, to the same strength level might be $3000.
I think, but I don't KNOW, that a dana will clear the tank. Measure from the axle centerline of your 7 1/4, to the back of the housing,likewise measure your dana, and see if there will still be space.