So unfortunately, after doing a ton of research, we will not be having tailpipe flames on Wendy for reasons of safety (we live dangerously, not stupidly).
So the traditional method of having flames shoot out your tailpipe is to dump extra fuel into the exhaust and then use a spark plug about 6" from the end of your exhaust that will light the unburned fuel when you push a button or something similar. Two problems with this, we have a catalytic converter and a fuel injection system. The whole point of a catalytic converter is to burn unused fuel, so we would have to rip that out. The O2 sensor on the fuel injector would have to go as well, as it keeps your engine from dumping extra fuel without hacking the ECU.
So this leaves the alternative of pumping propane into the exhaust line after the cat converter. The problem with this is that the flame can chase back up into the propane tank and blow up unpredictably. Now don't get me wrong, we are all about explosions, but we prefer it when we can control them. So if anyone knows a lot about propane routing, let us know!