Are you talking front or rear? by taking the leafs out of the rear spring you reduce it's stiffness therefore reducing the spring rate. You might be able to find a parabolic rear spring which is slightly different to a leaf spring but from memory the hearld rear chassis uses the spring to locate the top of the rear hub so the softer the spring or more shims/lowering blocks under the spring will increase the +ve camber of the rear wheels and lower the ride height. For the rear shocks, measure the open and closed lengths and just buy some aftermarket adjustable ones (you don't need to spend much on these) eg Gaz or Protec etc
Best bet is to re-furb everything, have a talk to rimmer brothers and see what they suggest (suppliers of all things britsh
they look like they do an uprated front spring and shock package) then suspension setup is usually set by feel once a baseline has been set. The hearald, like the mg's and tr's always benefit from a concrete block in the boot...
I'll try and find a book on triumph tuning i have at home and scan a few pages in