Well the shackle was the factory solution to service complaints about cracking with the center mount style. Here is a description of the problem located here:
http://auto.howstuffworks.com/1947-1955-chevrolet-trucks2.htmThe cab itself, which had previously been attached to the frame at three points via shackle bolts and rubber mounts, was now mounted at four points. The idea of the three-point system was to let the frame flex without affecting the cab. This worked well, but the single forward mount put too much stress on the front frame crossmember, so two smaller shackle mounts were set on the forward chassis rails instead. The cab was still unaffected by frame flexing thanks to the lessened tension of the shackle bolts.
Frame flexing had caused other problems as well. There'd been complaints, for example, that the 1947-1948 radiator support in the 3100s tended to twist and shake loose after many miles on rough roads. This caused splits in the radiator core, and cracks in front-end sheetmetal. For 1949, Chevrolet added X-bracing to the radiator support frame, reinforced the bottom channel, and mounted the entire assembly more rigidly.
In my opinion both is overkill. The issues outlined above were for the time they were built and used. For most of us our trucks will never see the use that caused these problems. I drive my truck 1500 miles a year in good weather, on good roads so I think either system will serve you well.
