New build questions?

Hi all, I am new the the forum. Thanks in advance for any help you all may provide. I have a smokeless encore now shooting 270 Parkers and happy with it, but I am putting together a different muzzloader to reach out a little further. As far as components go I already have an origin long action mag, grayboe stock, HIS 5/8 breach plug with alum. bronze modules, apa fb brake, load through funnel, and my local Smith is getting me a 1.25 straight bull bartlein 18 twist. My goal is to push Kyle's 328 or 353 hard-cores into the 3000 fps. Would love to do some 600+ hunting. From everything I have read already on here, H4895 sounds like the right powder for me but I have not been able to locate any. Jeff recommended a powder chamber cut to hold 80% of my powder charge. With that being said my biggest questions now would be what powder is best and how large of a powder chamber should I have cut to try and meet my goals, without going too big and have to always run a huge powder charge. Thanks again.
Taylor