talking from experience here, DO NOT go too stiff on the rear end.
I fitted alloy subframe bushes, 80shore rubber rear diff carrier bushes and tigersealed up the front diff mount, and the result was the thunk turned into a horrible clank as you went on and off the throttle. Even engaging the clutch between 1st/2nd/3rd when driving like a granny sounded like you were trying to snap the diff out the bottom of the car.
What happens is all the driveline components begin to wear, CV's, Propshaft joints, diffs themselves etc all end up with a bit of slack in them. All of this wear means theres slack in the drivetrain, which at the moment is being absorbed by the nice big soggy diff mounts.
I ended up swapping the rear carrier mounts out for new standard ones and that made it much more bearable. Fixing a worn CV on the propshaft helped too. I also changed the gearbox which improved it a bit more as well.
I'd suggest new OEM rear bushes (they're only about 8quid each) and filling the front bush with polyurathane sealant is a decent compromise, stiffens it up but leaves enough slack so that it doesnt transmit horrible clanks into the floor.