Gear ratio help

I just picked up my new 71 Valiant with a 225 slant 6, auto. I believe I have the 7 1/4 rear end and it howls like a mother at 70.
Since its nothing special Im not gonna fix it just swap it out. I have sourced several on craigslist for cheap. Can anyone help me with what to look for to make sure I get one thats a direct swap, ie: gear ratio and if they changed during specific years. The cleanest one I am looking at came out of a 70 dart but I think it was a v8 car. I am also planning on taking the springs since one of mine sags an inch. This valiant is just a toy and not a speedy car.