This is part of the “I don’t keep secrets” deal. The following drawing shows all of the pertinent block dimensions for the block adapter. The needed dimensions for the transmission are not shown so you can choose whatever bellhousing you want to work with, assuming that it will be physically compatible with the block in terms of starters and such. If you have the machine tools to make the part then it should be relatively easy to pickup and overlay the bellhousing dimensional data. The final shape will be determined as much by the bellhousing as it is by the block.
If you have the necessary time and tooling and think you can successfully complete all of the details then go for it!

Some information is omitted simply due to the final needs, such as the actual shape. Some will require ‘ears’ on the sides for mounts and others will not. Some will have tighter coverage at the center for block protection, others will not.