perkinsm,
Here are a couple of pictures showing the muffler installation on my high mileage UCD46 and the wrench that I use, which is a 14mm thin wall deep socket.

Put one nut on the threaded pipe (without the muffler) and install it in the engine's cylinder. Pass the deep socket through the hole in the cowl and torque the nut. Now put the other nut and the muffler on the pipe and torque the nut with an open-end wrench. The nut is outside the cowl so there is no need to make any other holes in the cowl.

As you can see in the picture the pressure line exits the cowl through it's own slot.
The engine is a Saito 72.