The internet is such a wonderful place.
I followed Derek's advice and googled 'Volvo 1800ES conversion to aw70/71' and found this absolute gem:
https://www.v1800reg.org/pages/AW70%20CONV.PDF
Not only has exactly the swap the OP was asking about been done previously, but the correspondent has detailed the entire process from research to driving the converted motor car.
It would seem that everyone was right about bolting an AW71 auto box (with a bell housing designed for a slant 4 motor) to an upright B20:
- the bolt pattern is identical, so the gearbox will bolt on, but would be inclined at an angle. This would work as such, but might lead to reliability problems in the longer term.
- the above may be fixed with a pretty simple adaptor plate. It would be something of a miracle if any of the batch that were produced 25 years ago were still available, but getting one CNC cut from the drawings would not be difficult or particularly expensive.
I think the lovely article in the link probably brings this answer to the OP's question to a very comprehensive conclusion. It has been an interesting (and fun) discussion and again shows the value of this excellent forum.
Good fortune OP. Now you have all the information why not find an AW71 auto box (pretty common and not expensive), have an adaptor plate manufactured and document the conversion to your motor car with some words and pictures in these pages?
Alan