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There is no one correct answer, but I'll try to give you some info to mull over.
First, understand that adding roll stiffness (which is what sway bars do) inherently decreases cornering traction. All else being equal, if you add a heavier front bar, you increase understeer (a.k.a. pushing). If you add a rear bar, you increase oversteer (a.k.a. fishtailing).
But all else is not equal, as is usually the case. Amazons came new with bias ply tires that were not capable of generating much cornering traction, and would slide before lateral forces could exceed the limits of the suspension geometry. Volvo's small front bar and no rear bar were well matched to those tires. Today, even the cheapest radial tires can cause the car to roll well beyond its intended limits, so you do need more roll stiffness to prevent the front tires from canting inwards under the car, which produces severe understeer.
If you add just a heavier front bar, you'll improve that situation somewhat, but you'll still have some of the inherent oversteer from the increased roll stiffness on that end of the car. It's easy to compensate for that by either 1) adding a rear bar to increase oversteer, or 2) using some negative camber in front to eliminate the understeer, or at least decrease it further.
The problem with adding a rear bar is that it makes the whole axle roll with the body -- that bar wants to pick up the inside rear wheel and tuck the outside one under. With both bars on, you are likely to have neutral steering under steady power, but the rear end will tend to kick out as you add power past the apex of a corner. It limits how soon and how hard you can accelerate out of a turn.
So, whether you'll prefer just a front bar (with negative camber), or both bars depends on your driving style, how much power you have available, whether or not you have a limited slip differential, spring rates, how sticky your tires are, and no doubt much else. My personal preference if for the heavier front bar only, and some alignment changes to make it behave. But that's me...
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