You are right about the pass car left hand manifold running into the steering box.

I've seen using the stock left hand exhaust with a big sweeping curve for duals, too. I always called that a "Gabriel's Horn" : the thing looks like a refugee from a tuba factory.

Addtionally, you'll note that those stock manifolds are not particularly generous in the passage size. '57 car manifolds look like they'd flow much better than a Gabriel's Horn, tho I'll readily admit that I've never flowed either setup nor have I seen flow numbers on them.

Using a right hand manifold on the left side was a path I'd considered, too. However, using measurements from the manifolds that I had, I found that the right hand manifold on the left side would pass much closer to the fuel pump than I felt comfortable with, and some trimming of the inner fender seemed to be in order, too.

If you are switching both sides (putting the left manifold on the right side, too) I'm guessing you will also be facing issues with pipe to generator/alternator clearance.

My choice worked like this:

1. Crossover manifold & Gabriel's Horn: cheapest, ugliest, probably worst flow. Puts a lot of pipe surface under the hood, adding to the heat already there.

2. Right hand manifold on the left side: adds the cost of the manifold to #1 above, less heat under the hood, looks cleaner. Some clearance issues with the fuel pump & inner fender.

3. Headers: more expensive than #2, (but not so much as one would think: bending the custom pipes for #1 & #2 will be more expensive than the straightforward hookup of headers, offsetting the cost somewhat). Better flowing. Maybe a bit noisier than cast iron manifolds and perhaps a little less durable.

4. Ram's Horns: Most expensive. Works most like a stock setup (essentially, it is one). Probably the same flow as '57 pass car manifolds -- less than headers. (I went this route. Paid $100 for my Ram's Horns and thought it was robbery! But, the price was competitive with headers at the time.)

So, it is sort of a "pays your money and takes your choice" situation. If all you care about is having duals, #1 might fill your need. If you want something "cleaner," maybe #3 or #4 would be more in order.