Just to throw a quick possible curve ball your way, which you can ignore or ponder:

Instead of the metal BB shell, make an over-sized one out of a block made from wood pieces, glued as [MENTION=61707]squirtdad[/MENTION] mentions above. Holes could be bored for the tubes, stays, and BB.

The size of the BB hole would depend on the type of BB and cranks used. At the minimum bore it to one piece cranks size and use a Euro Adapter.

When all the holes are bored correctly then shape and smooth the BB shell to finish.

This would also allow you to securely glue and even pin each tube/stay in place. Something similar could be done at the head tube.

Just 2 cents from an excited observer. Henry, keep the creativity flowing!