It's quite a bit. In addition to the obvious (flywheel, flywheel housing, clutch, hydraulics, pedals, pedal housing, shift lever, wood around the shift lever, gearbox mount), there are some not-so-obvious things that are different, like the driveshaft, exhaust, rear cross member and speedo cable.
And finding a Stag gearbox isn't always so easy either. Some parts are common with the late TR6 box, but the input shaft, countershaft and thrust bearings are different.