Gonna do as Hawg suggested and load from the get-go. I'll be using Starline brass, which has thicker necks and is harder. I ordered the RCBS Cowboy dies, we'll see what expander diamer is when I get them. Hard cast 200 gr bullets are readily available in .427, .428 and .429 varieties so I'll find out what bore diameter is when I slug the barrel.
I'm thinking I should be able to get over 1000fps out of a 24" Bbl without exceeding any max loads.
Of course! If you can get some Green Dot you can obtain +1000 fps with the lowest load according to Lyman's reloading handbook