Isn’t the auto’ ‘box shorter than the manual o/d ‘box? The prop is certainly longer, and by more than the width of the manual adapter plate which isn’t used...
The auto’ transmission tunnel is flatter than the manual, especially to the rear, so you’d need to adapt that, but surely it would fit? Isn’t there a rear extension housing on the back of an auto’ box too? I used to have one I. The garden that I could have experimented with, but I gave it away
The 65 IS slighly shorter than a man/OD box, but only by an inch or two, but the loss is much greater than that as the BODY of the trans is much longer than the body of a non O/D manual and the tailshaft housing with speedo drive is correspondingly shorter, only about 6" long. Then there is the shorter mainshaft you'd have to make for the autobox and the adaptor housing too, I don't think it's a viable alternative.
Like Jeroen says, get a 4 speed box to start with......I did!
The Toyota/Mitsubishi Auto range nearly ALL have "O/D" but its not a true overdrive, just a 4th gear that can be locked out manually, its still operated by the same valve block as the rest of the transmission.