It's not just Jigsaw racing that can mod your diff.
As the diff. is shared between the small chassis Triumph range any decent Triumph specialist should be able to help. Rimmer Bros. and Canley classics are two that spring to mind.
That opens up the whole range of ratios too. 3.2 for the cruisers, up to
4.8 for the sprinters.
Jigsaw may be able to help with uprated half-shafts though. They advertise them for the Dolomite, but it may only be for the sprint and they may be different.
Looking at the front of the car, if you want to upgrade the hubs and stub axle,without changing the PCD, Canley classic do this kit here