Running a 3.55 rear axle gear with a 500 or 518 automatic transmissions will give an overall ratio just under 2.5, so you get the performance advantage of a 3.55 and the economy of a 2.45 axle. It's the best of both worlds! (running an 833OD 4 speed will yield a similar result)

Personally, I'd run the 500 trans if it's strong enough for your intended use. (It's based off the 904) These had lock-up converters and they had the lower first gear ratio. With the lower first gear ratio, I'd run a 2.92 or 3.23 instead of a 3.55 gear.