There are lots of different places the manifold can crack. Mine has got a hairline crack where the turbine flange meets the runner, and another smaller one where a runner goes into the head flanger. Anywhere the manifold is welded can split.

As far as structual support for a log manifold, there isn't much really, unless you use the stock downpipe bracket and make a bracket to connect your custom downpipe to it. That would help share the load of a very heavy turbocharger and all the assorted piping... but if the welds are crap to start with, this won't fix the problem.

If you are making it out of mild steel, using thicker piping (schedule 40) will help prevent cracking even with a long tube design. If you are making it out of stainless and are a competent welder or know a very good welder, you can make it out of anything as thick or thicker then 16 gauge.