OK I'm guessing and can offer this as a theory only. Those were relatively early days for alloy engines and porosity was an ongoing, huge problem. The alloys back then were so full of holes they wouldn't keep the water in the right places, same with the oil.

Whether that was the problem with the GM engine I can only speculate, but it certainly had issues with the mating surfaces for gaskets and oil seals.Suspect this was partly due to the greater expansino rate of alloy compared wtih iron.

In the end GM sold it to Rover who re-engineered it with particular focus on the gasket surfaces and oil seals (they were still modifying it when the SD1 series came out in 1976.) It has found widespread use in English cars, often replacing 4 and 6 cylinder engines and it could well have weighed less than the Anglebox 4 cylinder it replaced - the V8 weighed 144kgs.

It also found its way into the Leyland P76 as a 4.4 litre version.