Stewart,
The 40's flywheel holes are smaller and present a problem. Fortunately for me, the '55 235 I used came with a '41-'54 10-3/4 clutch flywheel. So I used that. As Dave mentioned, using the later flywheel is the way to go. Make sure you get one with the ring gear to match your starter (6 volt or 12 volt). While the 6 flywheel mounting holes look symetrical, they are actually made slightly different so the flywheel can only mount in one position.
I wrote up some lessons learned from the swap here:
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=010423#000000
Steve
