I would just get the chromo inner and outer shafts for the what the price difference is versus Spicer shafts nowadays. Also would not recommend getting used shafts because you don't know what the history is, and fatigue life plays a big role on how they will hold up.
On my rig I broke a stock neckdown inner shaft first, and then broke 2 30-spline stubs and twisted a third all at different times. Replaced the inners with Spicer non-necked shafts because at that time the only other option were very pricey chromos (probably 10 years ago). At a later date I went to 35 spline Yukon stubs. I have not broken anything since replacing the shafts and am using the same Spicer u-joints I started with (they survived the multiple shaft breaks). The other advantage to chromos is the ears on the shafts don't deform or the u-joints holes don't get egg shaped and wallowed out. I have two friends with that issue on Spicer shafts running 42's and big blocks.
