I have heard the shaft kit is much stronger, but I talked to a guy the other day that has been running the 501 or "wedge" kit for 20 years with no trouble at all. Supposedly the 501 or wedge kit gets loose and sloppy. Let me know if you find a shaft kit. MM doesn't make it anymore and I have not been able to find one used. I am still running mine stock instead of getting the 501. If it comes time for a rebuild before I have found a shaft kit I will go with the 501, unless I get a deal on a NP205