After Peter's post, I went to the shop and measured some stuff lying around.

Bellhousing adds about 6" to the length. M41 is roughly 24" long, so total length including the bellhousing is about 30". There are at least two different lengths of M41, but the difference is only about an inch.

BW35 is also about 24" long including the integral torque converter housing, and so requires a drive shaft about 6" longer than the M41. I just converted an ES from BW35 to M41 and required front shaft was definitely shorter. Auto shaft could not have been used without cutting it back.

M40 is about 12" long, so total length including bellhousing is about 18".

I've heard numerous times that an automatic front shaft could be used instead of shortening the original when converting from M40 to M41, but I've never had one on hand to try it when doing that job. From what I measured to day, I don't think it would be even close to possible. ???