Can I convert a 1969 b-body 833 with the single mount for shifter to a 1970 E body just by switching out the Tailshaft???

If they both have the same rear bearing in the tailshaft you can

Yes the 69-70 b-body tail housing will interchange.

Yes. You might as well rebuilt it while you do it. As Cab said, the bearing in the tail housing can be of a different size. The Hemi trans used the bigger 308 bearing so the bearing and snap ring are different. When you unbolt the tail shaft, the main shaft and 1st through 3rd come with it. A service manual will show you all the detail.

This ^^^

It all depends on which bearing is currently in your B body tail. If your trans is 18 spline it will have the large 308 bearing, if 23 splinre , no it will have a 307 bearing and you will have to swap over to the 308 bearing unless you can find a B/E smallblock/383 small bearing Tail ... they are other there.

This is correct, however, IF you do, be aware that there may be some 308 bearings out there that are not the correct thickness. This came up a while back......
read here:
https://board.moparts.org/ubbthreads/ubbt...html#Post135794

jess