Steve
If you don't feel comfortable doing it in .577 then
do the .500 or someother NE round. I would not worry about
what people say you can or cannot do. Mostly it's just
talk. I have shot numerous full 750gr loads in mine. The
guy I got it from never shot anything but full loads and
then some while he had it. The No.1 is incredibly strong.
The chamber wall thickness for the last inch or so where
the barrel is threaded into the receiver is not a problem
provided you cut decent, tight threads and your smith
knows what he is doing. It is a bit disconcerting to
look down at the loaded action and see a bit of brass rim
protruding above the breech block, but you get used to
that.
And, you can always send it to the Birmingham/London
proof house and be real certain.
Good luck whichever way you go
Raff