install teh disposal and reroute the pipes from it to teh wall drain. may take soem figuring, but the pipe layout can be done.

David's right, position the GD discharge to the right and put your trap on that side.

You need to be able to take apart the metal drain line -- so you will need some big wrenches, but it can be done.

I would adapt it to pvc just as soon as I could... such as where the el comes into the cabinet. PVC is much easier to work with.

It doesn't look me as though a disposer will be a good idea under your sink, even if you can manage to shoehorn one in. There doesn't appear to be enough height between where the drain line emerges from the back wall, and the bottom of your sink.

If the bottom of the grind chamber and the disposer discharge tube is lower than the drain line, the disposer won't discharge waste and drain properly. Even if you run the disposer long enough and pour enough water down the drain to clear it of food waste, you'll still always have water sitting in the bottom of the disposer. In the short term, it will function, but unless the disposer drains completely after each use, it's going to have a short life.

it looks to be enough room. single sink, so it won't require the drop piece before a tee.