Thanks you very much for your offer of help,
I presume the 3.8 and 4.2 M116 engines are larger versions of the ones that Hatch & Sons successfully put in their cars.
They are. They have the same overall external dimensions. But they also have an aluminum block instead of cast iron, so they are a lot lighter. The M117 is also the same engine, but with higher block decks due to longer stroke. They are around half an inch higher and an inch wider overall compared to the M116. They also use slightly bigger diameter exhaust manifolds, so the clearance with side rails is limited, but it clears. The alloy block M117 is 43kg lighter compared to the iron block M117. I would imagine the alloy M116 to be around 5 to 10 kg less than the alloy M117. The alloy M117 is around the same weight compared to the M130. So the M116 will be lighter. Further, the center of gravity of the engine is lower and more backwards, so it improves the car's overall balance.
Hatch& Sons claim the iron M116 is only 29lb heavier than the M130. I think it's heavier than that. May be 30 kg. They also made some specific engine arms. I would use the stock 3.5/4.5 engine arms, like I did on the Coupe. They do bolt on the alloy block. Smeone in Danemark fitted a 3.5 in a Pagoda keeping the stock engine arms, he was pleased with the result. I don't know why H&S made some specific arms.