65 Valiant 273 dual exhaust

The biggest performance exhaust upgrade you can make is mandrel bent pipes. Even with a 273, you can pickup 15+ hp over crimp bent pipes you'd get at a local shop. A crossover will help as well but not as much as the pipes. An x could give you another 8-10. The H help but only on the bottom end torque and at heavy throttle, it sounds a bit more traditional V8 sounding whereas the X starts to have a turbo type sound.

If your motor has been worked with a bigger cam, these number will increase significantly.