If you really just want to use the newer wheels, you have two options. One is a bolt-on solution that requires access to a later-model 1800. The other option involves finding a skilled machinist and risking your hubs and rotors.

Option 1.
Take the hubs, rotors, and calipers from an 1800E or ES, swap them onto your 131. Drop the rear axle from the 1800 E/ES and install it in place of your original 131 rear. With a 69/70 model you probably already have the nifty dual-triangular circuit power brakes (that I so love), so you'd just have to "MacGyver" the e-brake cable to make it work. This is by far the easiest way to go if you need to stick to the bolt-on upgrades. This "upgrades" your wheels to use the later style - 140/240/740/940. It also gives you modern disk brakes all the way around your car. However, it can be hard to find an 1800 worthy of sacrificing...

Option 2.
Remove the hubs and rotors from your 131. Take them to a machine shop. Have them plane off the old wheel studs and *carefully* cut new stud holes in the later bolt-circle. This gives you access to any of the later rear wheel drive wheels including those nice wide ones you have.

This doesn't solve any issues in your old rotors or brakes, but it'll let you get some newer wheels on there.

Someone may correct me, but I think the 140 uses the same brake pads as the 120.
I think the rotors are the same size as well...

Also, if you are not afraid of cutting and splicing your steering wheel assembly, you can make that 140 box work. I only recently explored the innards of a steering box. it is fairly simple in there... However, you might be able to rebuild you old steering box using parts from the 140's box. If the parts are interchangeable, it would be much simpler than retrofitting the 140 steering assembly.

The steering assembly from the 1968/79/70 models are the safest available. The 140 type is no real improvement on it. Try and save what you have if possible.