biggest area on the rover V8s in the head - a set of stage one heads should liberate around 25-30hp when you retune the carbs.
Stage 1 Cylinder Heads - V8 Tuner
headers and a decent exhaust system will release a few extra ponnies but nothing that you'll really notice. changes to the ingnition will only work if your original stuff if ****ed.
if your car is high milage then the cam is probably worn so even fitting a new OE cam will make things better..
the other alternative is to junk the 3.5 and fit a 3.9
Rover V8 3.5 to 3.9 conversion. - V8 Tuner
sadly to get anything really more out of the engine you will have to spend a bit of cash