Tim, not to be a smart***, but let's get some terminology correct first. Backspace refers to wheels. If you lay a wheel face down on the garage floor and lay a 2 x 4 across the rim on the backside, then measure from the bottom of the 2 x 4 to the mounting surface on the backside of the wheel, that is the backspacing of the wheel.

What you're looking for is differential width or rearend width or rearend housing width, either from the wheel mounting surface on the axle or the wheel mounting surface on the drum or from the backing plate. It should be a simple matter to measure the width of the housing on the '49 and on the '70. Compare them and see if they are close.

I would think this would be a natural for a donor car. I'd take everything off it including the electrics.

The rear in the '70 is a 7 1/4" ring gear, 2.76:1 standard ratio.