1) Making a '51 mother board for any purpose other than temporary use as a development platform is futile. The demands of what you might want to attach is way too variable.

One thing I have for development purposes is a board with a '51 and a CPLD. A connector is fed 40 unassigned CPLD pins and P0 and P3.

This has in a very few cases been helpful, usually it is far better to start from scratch the result of using the above often is too far from reality.

Erik