Thanks guys! I have solved my problem. Nice day here in Maine today. Temperature actually went up to 35 -- ideal for junkyarding, and I was able to swap my 3 3/8 flange for a 3 7/8. I didn't even have to dig around in the snow for it.
Matt,
Using the AT flange was my first thought. Turns out that they attach with a bolt rather than a nut, as on the M40 and M41.
Bob S.

On further inspection i have found splines are other way round m40 fine spline, i assume m41 coarse. Pardon my confusion, i am becoming an amnesiac.

I'll assume that you've checked and confirmed that the flange from the old auto box doesn't fit...?

You might also check with your local driveline shop... Might not be an entirely uncommon item. I've probably got one somewhere, but not here at home, so it would take me a little while to dig one up (if I've got one for sure). Would help though if you could give me a spline count...

-Matt

When i swapped auto for m40 i modified 265 shaft and used 240 diff flange on m40. This gave me 2" shaft two large flanges, three large uni joints. Have since found out m41 has multi spline flange and wont fit m40 with coarse spline. You may be able to use 265 auto flange if splines are same or have one fabricated using fine spline centre. Regards Jack McIntyre.

You might also look for a short front shaft with the smaller flange.
In fact you might get the flange off another shaft and just install by
changing the U-joint. (unless the u-joints are different too)
anyway it pays to look parts over pretty closely. A lot of unprescribed
things will work.
--
George Downs, Bartlesville, Oklahoma, Central US