With 400' rolls I always used lid of the film can as "tray" and from there I loaded on the spools. Yes, you will have 2 or 3 circles of film between big roll and your spool but there is nothing to worry about. Just be sure that you cleaned all the dust and junk around your working space. If you keep 400' roll vertically with out same kind of flanges, you will be in trouble. As for light tightness, just do it during the night. Plastic core has hole just over 1" or precisely 26mm wide. Good luck!
It never occurred to me to just unspool it from a horizontal position. Thanks for concurring on the danger of vertical use!
Henny Youngman: "Doctor, it hurts when I do this."
Henny Youngman: ...
I photograph things to see what things look like photographed.
- Garry Winogrand