Hi Greg... Is it the LT77 box you refer to?
If the rover box goes straight onto the montego block then the alternative would be to buy a bellhousing adapter so that you could mate a ford box to the montego block. (Instead of the usual rover v8) That would give a better choice of boxes and ratios etc.
Mention of rover boxes
HEREThe triumph stuff I know little about so can't offer any advice :(
Good hunting! :)
G