Welcome to the site!

At my last visit to Mark Fore and Strike, they had a baikal 45-70 double rifle in stock. Done, built, good to go, no modification necessary. Good deal on a good rifle.

Building a double rifle, either on an existing action or completely from scratch -- is a very big undertaking. Spend your money wisely on a running rifle, then if you find yourself with a few extra thousand dollars and a great deal of time (when you're not working/studying/sleeping...) to throw at such a project. You'll need to come to the effort with a great bit of skill and tooling too.

Cheers from Palomino Valley!
Tinker