Hi dliva,
I am not familiar specifically with a Steib but the principals are the same. Some plunger frames utilise castings on the frame for side car fittings as well as substantial engine bolts. M21 and A10 for example have integral fitting in the front just under the head stock and use the rear footrest mounts.
I think that your frame does not have these but probably the rear footrest mount is usable as well as the lower/front engine mount bolt and the frame bolt under the rear of the fuel tank. Clamp type fittings can also be used. Basically 4 points are used although a fifth one will stiffen things up if needed. Do you have a photo of the Steib?
If you google 'motorcycle sidecar fittings' you will images of various setups and sidecar fittings.
Dave.