From what I understand, the wagon rear control arms are different from those on a sedan, so you can't just bolt in a sedan rear end. There are also differences from year to year, which may or may not matter (it's been a long time since I had my 122)

You might want to contact Repairmanjack--he put an 1800E rear end in his 122 wagon

If you have a Dana 30, you should be able to swap in the ring and pinion from a 240 or a 740, giving you a better selection of ratios. It seems to me that a 3.73 would be even better than a 4.10, especially if you have a B20, with its additional torque. 3.31 would be better still, in terms of gas mileage, but I don't know if the engine would be able to pull it, especially with the car loaded down. If you try the 3.31 you will also want a late (73-75) transmission, which has a shorter first gear. That might turn out to be the ideal setup---I would guesstimate 28 city, 36 highway if your engine isn't too wild.