building a tr6 OD transmission

[dombey@plato.ds.boeing.com (John R Dombey)]

A query to the wisdom of the net:

How easy/hard/difficult/impossible is it to take an early TR6 gearbox,
(I've got both a '69 and a '70 box) and mate it to a J-type overdrive?
I've heard that converting a box to overdrive is not that hard, but is
that only conversions of with similar vintage parts?  Anyhow, I've come
accross a J-type for a reasonable price, and was wondering what I'm
letting myself in for.  All replies welcome...

John