Need some help and advice on rear axles

Hello. I need some advice on rear axles. I’m an amateur whose been working on a 1970 Plymouth Duster. It’s a low budget project, originally a slant six car. I bought a 273 complete with 727 auto trans for a bargain of a friend. Did the swap this winter and it all turned out really well I’m happy to say. The little 273 runs great, but I still have the old 7 ¼ axle on the car. I’m not sure of the gear ratio, but it is really sluggish, nice on the highway though. But it won’t last long I guess.

I´d like to have a lower gear ratio so I can take of faster. And some sort of diff-lock and something that will last behind a smallblock. I suspect I might upgrade to a 340 in the future.

The a body axles don’t show up so often here in Sweden. So far I’ve found these ads (condition “should be fine” according to owners)

8 ¾ 741 housing 3:23 ratio, diff lock = 2010 USD
8 ¾ Formula s 741 housing 3:23 ratio, no diff lock = 1055 USD
8 ¼ with 3:21 ratio, diff lock = around 1290 USD

What should I look for/think about hunting axles? Would you go for any of these?

By the way, here is a video of me with the duster on the strip for the first
time in my life! Lots of fun so it won’t be the last!
[ame="http://www.youtube.com/watch?v=jdS3BKo77pk"]YouTube- 1970 Plymouth Duster 273[/ame]