I too have had a late series car with D type over drive and fine splines on the gearbox input and output shafts.
I have a later series o/d box in the garage which has fine splines, a longer inpup shaft/bellhousing (which can be changed) and also the spacer between the box and overdrive is slightly longer ( about 1/2 inch). It is a 25% ration increase.
I have the yoke for the propshaft and comparing to a standard overdrive box, it looks like the difference in the spacer collar would be accomodated by the splines in the propshaft.
I will let you know if I ever get round to fitting it !
regards
-- Jeff Howarth