Well I have more or less the same configuration on my plane but with a TT 120Pro and bisson pitts. Now I can't seem to agree with the 3rd line there, It is useless. There should be 2 lines coming out of the tank, one that goes to the fuel valve, and then to the carb, and the second one on the muffler. If you have another line out of the tank than all the pressure from the muffler exits from the 3rd line instead of going to the main line to the valve.
Now I read that you say that you seal the 3rd line once the tank is full correct? Well that makes a little more sense, but why have that line in the first place; to see when the tank is full you can see it when the gas comes out of the muffler.... this won't affect you in any way at all.
Last thing is this, yes crimping the muffler exits a little bit will make your pressure problem disappear, I did that on mine also, but I also have a fuel pump, just to be sure, but first try to eliminate that 3rd exit and if it doesn't work than crimp the muffler, not too much though...
BTW I did the crimping on both exits of the muffler