Looking for some Differential advice

Hey, thanks for any help! Here’s my deal. I have the Dart in my pic. Right now it has a 225 and a 904 trans with a weak 7 1/4 rear end. It bogs out climbing hills. I have a whole new drivetrain for it. Here’s my question. Would I get anything by putting my 8 3/4 sure grip 3:73 rear end in first? Right now it’s a beautiful slug.
Oh, wtf happened to our site?