Dana rears come in different widths, if lug bolts are long enough, duals can be fit on both. Perhaps Grigg will chime in, he knows all. I just put an 80(70 would be like) in my ‘55 GMC, that fits a 7’ bed, probably C and C unit, other units fit an 8’ bed.

With some creative fitting, older, narrower springs can be made to fit 70 or 80 axles. 80’s are 4” OD; to fit narrower spring perches, get a suitable piece of 4” OD pipe, get a suitable piece of, say 2” wide steel, half the thickness of the diameter differences, lay the perch in your press, place steel plate on top, put pipe on top, squeeze pipe into perch with steel in between, OK to have steel too long to start, 2” steel fit mine, cut to fit length. Have to adjust welded perch on 80 to allow spring bolts to fit, get appropriate angle shims to adjust properly, make sure spring bolt heads engage perch properly, make extensions if necessary.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.