During the 10 years I owned a driveline shop we learned many things along the way. Fortunately none of them were the hard way as we were always extra careful. One of them is that the length of the splines on a slip yoke does not necessarily mean that the transmission will accept this amount of slip. Some may but other do not because while slip yokes are made in a variety of series, lengths, diameters, number of spline, etc. the trannys seem not to always follow what is available. In other words a yoke may have 5" of spline length but the transmission may only accept 4" of it. This is important information because I've seen shafts with the incorrect short yokes simply because it was available. If this were to bottom out it's likely that both the driveshaft and the tranny would be damaged. This is really important on any used vehicle since you never know what your dealing with. If the transmission info is available to a good driveline shop they can figure it out for you and build accordingly. As a general rule of thumb I would recommend 70% of the length the tranny would accept as the proper ammount to go inside the slip yoke. Whenever there was doubt this rule never failed us.
