Thank you all for the responses. This is encouraging although not surprising. I retired my '85 240 wagon and loved the O/D trannie so much I saved it for a project in the garage.
well it's good to know that at least the overdrive unit has been done before. I see what you mean by major surgery for the use of the gearbox as well. Thank you for comments!
FYI, the unit is a Laycock J type and has a small switch in the gear stick -super convenient! I imagine that other people have changed the set up so that it can be used in all gears.