"But, you could convert it into a .577; sell it, as truly big bores bring bigger money than smaller bores, which would give you more than enough money to buy most any smaller caliber double rifle, more suitable to your real needs and wants."

If you are doing it just for the money, then
IMHO it is the wrong reason.

But you are still going to have a home made 577
and unless it is a full Nitro 577, the action
is too big and even then it is probably still
on the large size.

Why wouldn't someone who wanted a 577 Nitro
just buy a new one ?

And for under $10,000 you can buy a vintage
Toplever Hammerless 577 Nitro for Black gun
like a Webley. Just look what is available.

And if all you wanted to build was a 577 Light Nitro,
then just buy a Greener 12 gauge and build one on that.