Your info looks like the stuff from the Mopar chassis manual. There is a small mistake in that that could influence your decisions. The 68-70 b body rear housings are wider then the earlier B body housings. Not a huge amount, I think it is only 3/4" or so, but it may be enough to make a difference in a swap. The spring perch location is the same, however.
Before you start swapping and/or cutting rears, have you checked the wheel well size to see how much tire you can get under there? Unless you plan on adding wheel tubs, the inner well and frame rail will limit tire size more than axle housing width. With the variety of offsets available on wheels these days, getting something that tucks up under the lip of the opening may simply come down to ordering a special wheel compared to fabricating a new housing.
