IMO there are several things that you need to re-consider with this project.

#1 Just because the case will accept 5 drams doesn't mean that you can fill it up that much safely. Wads have a purpose.
That old rolling block won't handle express rifle loads. They're not that strong. Stick with loads that they used with black powder SG shells.

#2 a .620 barrel will use a .621 or a .622 diameter bullet not a .640

I had a RB 45-70 that had an 1 1/8" barrel on it and it was very heavy. It was 30" long IIRC, a Navy Arms conversion I believe.