The Triumph Stag could have used the Rover 3.5 litre engine but they decided to develop their own. The big problem with the Stag was the reduced number of cores in the radiator which caused the engine to overheat. The other cooling problem was that when the Stag was serviced the cooling pipes were never cleaned out properly so the water flow was restricted. Apart from overheating the heads warped so they needed to be skimmed. The worst engine, I don't think so. Have a look at the Hillman Imp. Wrong engine in the wrong place.
As I recall the the Rover V8 was built under licence from Buick, back in the 1970s the Range Rover sold rather well and the MGB GT V8 (I owned two and grew up in Abingdon where they were assembled) used the same lump in a detuned low compression form (same as the RR). The V8 MGB was killed off after just 2591 were built so BL could build more Range Rovers..so the Stag never had the Buick engine option in reality.
The Stag cylinder heads and skimming - not recommended and rarely ended well!
Some 25+ years ago I was working with a forklift company called Kalmar who had bought the Coventry Climax business in Sandy Lane in Coventry, the IT manager back then took us into the basement where there were several large wooded cabinets containing engineering blueprints....yep the Coventry Climax engines that eventually ended up in the Imp.