The seller is refering to the boat as a 60/80. The first Eagle SG that we built was a 67/80 boat. It was 41" long. I think it was 3.8" inside. We later made a smaller boat for the 67 engine. It is 39".
When we built the first Gas SG is was based off the 80 boat and was 41" long. As to why the sellers boat has a gas cowl. Maybe he was planning to put a gas engine in it. I do know someone who is successfuly running a Zenoah in that original Eagle SG 67/80.
So to anwser your question in short. Yes, our mount will fit in that boat.
One possible problem is that the sides are higher than our gas boat, so the carb won't clear the deck without notching the side. We used light ply to build those boats and they depend on the continuous deck for strength. If the deck is cut the boat will be weakened and would surley break where the notch is cut. The possiblity of adding carbon fiber could solve that problem.
As a referance our current Gas SGX is 43".