Not worth the effort. The output shafts are different - that means you need a complete 'donor' M41 anyhow - and you'd have to pretty much *completely* disassemble both boxes to swap that. And the cases are different - although only in that the M41's fill hole is slightly higher. Based on the fact that you need an M41 anyhow, you might as well use IT instead of any M40 you have. As far as swapping various parts between an M40 and an M41 - beware the various brealy perceptible changes over the years. Other than the much longer output shaft they are the same, but from my experience the odds of parts swapping between any two M40's or M41's is not that great. Check part numbers before trying any interchanges. Syncro cones don't seem to have changed much, but gear sizes, layshaft sizes, and roller bearing sizes all made various changes over the years.

A couple of winters ago I tried fixig my junkyard M41 (nice gears and OD, but syncro's #1 and 3 were totally non-op) using my original M40 for parts. After some carefull study I found that the ONLY parts that would interchange were the syncro cones. Luckily, that was all I really needed.
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I'm JohnMc, and I approved this message.