if your looking to get the most tire clearances from the relocation, mount the front spring box outer side to the inside of the outer sub frame rail, don't center it
I'm not familar with the rear spring hangars on that year B body, do they unbolt? If they do you may be able to cut a V in the rear sub frame and weld that hangar into the sub frame. See the picture attatched to see what I'm talking about. For locating the front spring bolt location see if you can buy or borrow a machinests transfer or center punch the same diameter as the bolt. remove the leaf spring and leave the stock hangar in place, slide the center punch(transfer punch) in an bang it into the rear sub frame, use that as your locating marks, do both left and right sides
the trick is finding a 90 degree drill motor and short enough drill bits to use in that location after marking the location
Greenlee makes chassis punches in a lot of different sizes that work excellent, there a little spendy for one or two times use but they cut a very precise hole, they do require a pilot hole to start with