You could try H-335 powder with 52 or 53 grain Hollow point
match bullets first. I've had them shoot very well in barrels with
a 1 in 9 inch twist. If the rifle is going to shoot small groups, it
may well be with the above mentioned stuff.
You didn't say what bullet weight(s) you were considering, so
I tossed those out as a starting point.
As for how much powder, I would rather you look at some
online data available from Hodgdon Powder, instead of quoting
anything like that here.
http://www.hodgdon.com/data/rifle/223rem.php
