I use slip-fit lengths of K&S brass tubing over the short exhaust tubes that come with the FT-160 (and I assume FT-300). I don't remember the size but it fits like a glove. I anneal and bend the K&S to exit the cowl where I want it. The exhaust tubes on the OS point inward and downward so you end up with the K&S forming kind of a "Y" and I make a simple bracket out of flat brass stock and screws to secure the lower end of the "Y" to the firewall.
To make the vent line, I take a short piece of K&S 1/16" brass tube (about 3/4" - 1"), cut one end on a 45 degree angle and solder a 1/8" or so piece of the next larger K&S brass tubing on the other end to make the equivalent of a fuel barb. Drill a 1/6" hole in the whichever brass exhaust extension tube you want vent line on. Roughen up and clean with alcohol both the exhaust tube around the hole and the vent tube, insert the vent tube halfway through the exhaust tube, with the slanted face of the 45 deg end facing forward. Secure with a nice dollop of JB Weld (make a healthy fillet both up the vent tube and on the exhaust tube).
I find it best to also put a 90 deg bend in the vent tube piece (before you cut it) so that when the vent tube is inserted into the exhaust tube, the fuel barb points straight back along the exhaust tube.
Doesn't seem like it would hold up well but I haven't had one fail yet.