Quick update that I've been meaning to post to this thread...
I eventually went back to the PT OEM exhaust. The replacement muffler I discussed in this thread wasn't really meant to handle the type of installation necessary on the PT. The tubing is too thin and doesn't support itself well enough. It was really meant to be mounted right to the engine block itself with the attached bracket. The main problem I had with the stock exhaust was heat. I felt the squared off tubing in the manifold inhibited flow. Since upgrading to the Robin EH72FI, there is even more heat in the engine bay. Between the increase in horsepower and now having an oil cooler on the exhaust side (EH65 didn't have this), I was getting vapor lock pretty badly when it was warm outside and I was working the machine pretty hard (e.g. digging or mowing). I decided to move the fuel supply line to the other side of the tub. It runs above the battery to the back of the tractor and up to the top of the engine. The fuel pump is inline, and there is already a bolt on that side to hold a bracket for the wiring. I used it to also mount the pump. Since the line is no longer next to the oil cooler fins and muffler, I think I should be okay. I also have a fan mounted to the engine compartment cover that blows air across the engine and out the muffler side, which should help further with avoiding vapor lock. I haven't really worked the machine as hard as I'd like and the weather has been a bit cooler too, so I may not know for sure that it is completely fixed until middle of next summer. I have a one of those laser temp meters that I will shoot into the tub and see if I can find a difference in temperature on one side or the other. I'm fairly confident there will be and I should have less of an issue with the new routing.
Overall, the EH72FI is running great and I still love my little PT. I don't put as many hours on it now since I picked up a 61" Ferris ZTR mower. The PT wasn't a terrible mower, but it definitely doesn't have anything on a commercial ZTR when comparing speed and quality of cut.