Well, it can still be measured for correct fit.. much like Russ has described..
Take a day,(or some time), have the rims & tires ready or their exact sizes/displacement, put the Pinto on a lift so that it can be measured from the underside without removing anything except the current wheels. Measuring from center of the vehicle - out to where the new rim-hubs will rest, making sure there will be enough clearance for the whole tire and all it's vertical travel when they are in place,(if you are going to lower the back-end any, this should be considered within the wheel-well for proper fit without rubbing or you'll have to tub it).
If you are not totally comfortable getting these exact measurements to size & cut down the new diff, have a very, very good mechanic that has done some before, do the measuring and even the sizing/cutting of the rear-end. If whomever does the measuring/cutting down isn't willing to fully guarantee his work, take it to one that will. This is not something to play with & it simply must be done correctly, even down to the welds & materials used.
Now when you go to re-spring it, the drive shaft "spin-up" side is going to need to be arched slightly higher than the "spin-down" side, especially if you build the engine with quite a lot of torque in it.. this slightly higher arch will need to compensate for the engine's heavier twist off the line.. otherwise you'll need to build the rear suspension a bit too ridged to compensate for this, resulting in a stiff ride or always having to under-inflate the tires to soften the ride.
Make sure you adequately shock the rear,(I personally like 'Ranchos', they're strong yet still smooth enough to get a good ride & they seem to outlast many other brands)..
IHTH
Michael