The 3.5 is the outside diameter. I'm pretty sure I got a pair from skidim for my 92 supra and they were the exact same part for my 2008 Moomba XLV.

Yes, I'm pretty confident that your boat overheated once and that it warped the plastic. On mine, I ran one year and didn't tighten the screws on my water pump and blew the cover off. The boat got hot and went into protection mode and I realized what was up. Not hot enough to damage the engine, but hot enough to melt the upper elbows that go into the top of the manifold. Then over a year later I found the parts leaking like yours. I almost sunk!

I got the new ones and put them on. I also ran a handful of stainless screws on the inner flange so that the exhaust elbow had something to bite into (I made it 'studded'). The flange has a taper and the inside diameter of the hose is bigger than the outside diameter of the flange. The best success is to get the elbow as close to the hull as possible, secure it so it's biting into the studs, and then attach the top end of the elbow trying to NOT pry the elbow off the flange in the process.

I had to fight with mine a few times before I finally got it on secure for good.

Call skidim on the phone and they'll verify that their flange is right for your boat, I'm sure it is. If water's running though your exhaust, it won't melt in the future.