The rental dealer should be ready to help you size the shaft for what you need. If not, go to a different dealer. Is your life worth saving a few bucks? Most of the shaft length issue is based on the geometry of the 3PH lift system. If you have done the fitment right, you should be golden if you own the shaft. If you do not own the shaft, cutting it usually inflames some tempers and costs money. If the shaft is cut to length for a tractor that normally has a quick hitch system on it, it's going to be longer than one cut for a tractor with standard lift system. Quick hitches usually add about 4 inches to the effective length of the radius arms (lift arms) which totally changes the length requirements. Usually the added length of the arms is more than the shaft will tolerate if it's cut for a machine without a quick hitch, or conversely, more than likely to harm your tractor if it is fit for a quick hitch, and you don't have one.
You need to either own your own shaft for this, or make damn sure the dealer will help you check the fitment. If he gets a puzzled look on his face, call his competitor, immediately. Run away, fast.