The one I built back in the 80s has an Ed Rayle barrel on her...I ordered a .530 Rapine bag mold and have used that for the past 20 years or so...Earlier this year someone on here (I believe either Lemone or Semisane) mentioned that Lee had a bargain basement sale and their .530 molds were on there for about $20...I picked one of them up as well, have it in the basement and plan to use when the weather breaks...
As for patching, I use the blue cotton pillow ticking from WalMart for about 4-5 bucks a yard...I cut at the loading block or muzzle...I've never miked it but it the way the ball loads, I believe it's .018...
I started with 120grs FF Goex years ago, then went to 100grs FF and the last 8-10 years have ended up with 80grs FFF Goex as both prime and the main charge...
From what I hear, most guys use a .530 ball with either .015 or .018 patching...I use the .018 patch with a clean barrel or even for a dirty 2nd shot...After then, I have 2 balls with .015 patching around them for easier loading with a fouled barrel...
I'd pick up a box of .530 balls in the next few months and start with them, if they are too tight or loose, melt them down...There is also a guy that has been written up in one of the bp mags that custom makes molds and sells .526 or .527 molds for even tighter bores...
I might even have some old .535s in the basement...I might could be talked into mailing you 10-12 .535s, .530s and a strip of the WalMart ticking so you will have something to start with...
Another thought...With a new gun, I like to cut a strip of ticking about 10-12 inches long...I then start a ball, having the ball in the middle of the ticking, then you can pull the balll out with the extra ticking...
This lets you know how easily it loads and you can look at the ball for the marking left in it so you know if it's engaging the rifling...
One final thought...If the barrel profile is the same (I'm guessing you are going with a C weight)...I'd surely go with a .54...If you want a little lighter barrel (B weight) and it's offered in a .50 and not a .54 then you might want to stick with the .50...