A 65 Dart with a 273 four barrel motor would have had a single exhaust pipe down the drivers side of the driveshaft tunnel, in the factory configuration. If you try to run that with stock manifolds on a 273 that's making 365 HP, it will choke it to death. There are several threads and pics here on the modification that needs to be done to the passenger side of the torsion bar crossmember and transmission mount crossmember to keep that side exhaust pipe from hanging so low. I'm going to do the mods to my 65 Barracuda for the warmed up 273 going into it, to get true dual exhaust. My motor is going to the dyno soon, and I will faint and fall over if it makes 365 HP....lol. I know mine won't make that much, but it will be enough to benefit from true duals. A bigger single exhaust like factory was fine for 235 HP. If you truly have 130 more HP coming out of a 273......you need better exhaust to take advantage of it.