To a practiced eye, there is nothing much to a 2 bolt main cap. It's not rocket science by a long shot. The caps themselves are quite easy to make, so any competent machinist should be able to make a main cap that works.

It's a rectangular prism with a half of a cylindrical section removed and two holes drilled and one end of the holes spotfaced. Even a poor quality 4340 steel main cap is 50% more rigid and at least twice as strong as a gray cast iron cap.
So this question is kind of bogus.

As mentioned above, the important part of replacement main caps is the installation machine work. That is what assures the holes line up and are pointed in the right direction. That is what assures the bores are co-linear and the right size.

If I was a machine shop operator I'd charge more for steel caps than for aluminum or cast iron, because the high tensile strength steel is much harder to machine.

The BCR system appears to be well engineered and would be my choice if I were to need block strengthening.

R.