We were offered the option of Rover’s 3.5 V8* in Terriers, was that an option for you?
IIRC, a Terrier was used as a service vehicle for the Austin and Morris entrants in the 1968 London Sydney Marathon. It was V8 powered. It was decided to enter the truck into the contest, as it was traveling the full distance. It went on to win its class. I don’t know if it was entered by Leyland Australia or the parent company.
Edit: *Actually, I think it was the 4.4 version, from the Leyland P76.