Was looking on ebay today and noticed this picture of the TR8 with seats in the back. It does not look that bad and there looks like there is a bit of room in the back for the legs.
http://www.ebay.com.au/itm/1976-1993-Triumph-TR7-Grinnal-TR8-Conv-Factory-Photo-/370275481029?pt=US_Racing_Fan_Shop&hash=item56362375c5