I haven't heard of anyone swapping retainers. The only other option I can think of is swapping the bell housing, but that won't work for you because of the hydraulic clutch. The only other thing would be to get another bell housing for the 61 and have it opened up. Then you could do a fit check before you tear it apart.. You might also look into rebuilding the 420. I've read it's not difficult, but that could lead to problems too if you haven't done one. There's a great group of stovebolters here to help, and I can check the Inliners International roster to see if there is anybody in your part of Canada.
Larry
