IMHO. A parallel 4 link system is without doubt the best way to control a solid rear axle. The links must be parallel in the horizontal plane and preferably a perfect pantograph in the vertical plane. This will give no geometric interference as the body rolls and spherical joints can be used for all of the pivots. In the event that the top arms or the bottom arms are converging in plan view you will get geometric interference and if you use spherical joints you will break and or twist something. The links arranged in this manner become a part of the springing medium which is undesirable. To allow for the geometric interference compliant bushes must be used at the pivot points, this then allows undesirable movement of the axle under accelerating, braking and cornering loads. In the event that the links form a pantograph in the vertical plane the pinion angle will not change relative to the body/chassis. The longer the arms the better but the longer they are the stiffer they need to be to cope with the torque reaction of the rear axle. In terms of transverse location a watts linkage provides the better solution for a road going car. The pivot point of the centre link is the roll centre of the rear suspension. The pivot for the centre link can be mounted on the rear axle or on the body/chassis. A simpler but less effective method is a panhard bar as long as is practically possible. The roll centre with a panhard bar is the point where the axis of the bar intersects the midpoint of the wheel track. ie If the panhard bar is parallel to the ground (as it should be) and equal in height to the wheel centreline the roll centre is at the centre of the rear axle in height. A mumford linkage system is high maintenence and not really suited to a large sedan type vehicle, they are very good on lotus seven type vehicles as ross noble mentioned. In all of this what is very important is the roll axis of the vehicle. This is the axis thru the front and rear roll centres. It is this that will govern the handling characteristics of the vehicle. Assuming that the cg of the whole of the vehicle is above the roll axis- A front roll centre higher than the rear roll centre gives less weight transfer to the front outside wheel than does a front roll centre being lower than the rear roll centre. The amount of roll can be controlled by (adjustable) anti roll bars front and rear. Converting the rear of an existing vehicle you are limited by the front suspension and its designed roll centre. The original leaf spring setup roll centre height will be at the centre line of the rear axle. Whether tou use a panhard bar or a watts linkage I would suggest you start with the roll centre at the centre of the rear axle and adjust it up an down to suit your driving preference. You will notice an improvemant in handling just from the more accurately controlled rear axle. To understand all of this I suggest you go to a model shop and purchase some ball joints (plastic), some coil shock units, and build a small block model of a vehicle. An accurately located solid rear axle is a very good system, after all Jim Hall used them in his Chaparral Can Am cars
Hope this helps,
Trevor