you put the T fitting on the pressure line inbetween the fuel tank and muffler,then when your filling just plug the exhaust holes on your pits muffler the overflow will go out of the open line out of the bottom to your catch container.
I am going to start using a extra tank I have thats a good idea(alfredbmor).
then replace the plug in the overflow and unplug the muffler,or just take your fingers off the exhaust tubes and there you go, by plugging the muffler holes the fuel will be diverted to the overflow line and not to the muffler canister,on the photo I posted the line is just inside the cooling fins so its out of site but easy to grab and pull out enough slack to drain into a cup,when I am finished fueling, I just plug it and slide the fuel line back in,its pretty easy and takes little time at all.
the nylon T I used has barbs so the fuel/pressure line won't slip off as well.