Here in the US there are 2 ways to go. Taylor Racing in Texas sells a remanufactured rib case with dog gears?Webster? internals, and then Winning Circle in Ohio has gear sets for the rib case in BMC close ratios.
That is the path of least resistance.
To maintain a smooth case I had an old one re-machined for a larger lay shaft and gears machined to fit and welded to make the cluster. I believe they were also Webster gears. A local machinist did the development for me, he has passed. I didn't use it finding the Richmond the car came with ( Taylor is similar) really a superb unit. I sold the smooth case off to a bugeye racer.
Good luck,
#377
Stewart Smith
122 National St.
Santa Cruz, California, 95060
831-423-0732 bus & fax
831-423-4410 home
831-234-7315 cell