4HU in a Triumph GT6?

[RoverTR6]

The Chassis is large enough. The car is a Spit/6 with a Rover V8 and TR6 Differential. I have a 4HU from a '77 2500m and want to upgrade to the LSD. The trouble: the 4HU is too tall for the transverse leaf spring. I'm thinking of eliminating the leaf spring with a radius rod and use a coil-over shock like the TVR had. The lower swing arm is GT6+. It's currently 1,700 pounds.

The question: How many pound spring am I looking for?