I wouldn't chamber for 3" and use 2 3/4"

You will eventually bell the end of the chamber
and then the case mouth will expand into it and
make it damn hard to get out.

I had a 500 BPE Double that was like this and
ended up off loading it in the auctions.

Some people say it will never happen but over the
years it will.

Why not buy a 2 3/4" reamer and some cases,
2 3/4" cases will always be available, you can't
guarantee 3" will be IMHO.

BTW - Is he a gunsmith ? Doesn't the client determine what the gunsmith does as opposed to the other way round ?