For sure 70 B body works, but I think 71/72 and perhaps 68/69 might fit (pretty sure they do). Google Mopar rear end widths, there is a chart online someplace. As far as what you will have to do to install it, you will more than likely have to remove and reinstall the spring pads which will require a welder. Its not difficult depending on your skill level. Basically this is what you do;
1. you cut the pads off the new rear end, pay attention where the center of the pad is aligned with rear end tubes (I usually draw or scribe a line down the tube in both directions 6-8". You will need some new pads (check Summit or Mancini racing).
2. Remove the old rear end
3. place the new pads onto the springs
4. insert the new rear end onto the pads
5. center the rear end, align the rear end with the center line you created and the new pads
6. reinstall the U bolts and snug them up good paying attention to the alignment of the rear end and the pads
7. you need to check the pinion angle, now this can be a cause of concern and discussion, I install mine with 3 degrees of down angle with the rear end under weight (put your jack stands under the rear end if doing this on the ground or use screw jacks if on a lift) use a cheap angle finder you can get from Harbor Freight, they are magnetic and have a red needle that swings back and forth.
8. once you have it set what you think its good, tack the pads to the rear end tubes, do not fully weld them
9. hook everything up and bleed the brakes
10. take the car out for a drive, DO NOT STAND ON IT, just drive it normal and listen/feel for any vibration. I would say to try and roll onto the power some but not hard. You are trying to see if the angle is way off or not. If it is, you will know.
11. Now you have 2 choices,
A. you can weld up the pads if you are comfortable with the pinion angle
B. if you are unsure or timid, then weld up the pads a little more so you have really good contact, then take it out and drive it again and get on it and see if you get any vibration. You can do this in step 10, but its up to you.
Another thing to look at when you start down this path is your U joint size, there are a number of sizes used by Mopar over the years, you don't need the biggest ones (1350), 7260 and 7290 are the most common ones used. I don't know what driveshaft you have but given you still have the original rear end I suspect you have the small ones, that said the B body rear end will most likely have larger U joints, there are several ways to deal with this ranging from replacing the pinion yoke to using combination U joints. If the engine is producing any decent power I would go with the 7290 joints on both ends which might require you to get a new drive shaft.
Hope this is helpful and I am sure I forgot something that someone else will catch.
