To get a truly accurate dyno result your dyno test it should be done an engine dyno. Chassis dynos have too many variables that effect the end result especially if testing at low rpm with an automatic transmission due to how a torque converter works. An engine dyno operator can measure the torque at whatever RPM you want the test to start at. It will give you a truly accurate test result. The dyno will do the math that is required to determine horsepower since it's just a simple math equation. Torque and rpm are what is actually measured.
If you are trying to determine low rpm gains on a chassis dyno use a manual transmission in 4th gear or whatever gear has a 1 to 1 ratio. The parasitic loads the driveline produces will still skew your test results though. Besides that not everyone uses the same driveline as you do.
Btw since is your car an automatic the torque convertor multiplies torque at low rpm, how do you plan on determining how much gain is created by it. Are you planning on having the shop fine tune your carb to perfection before you add you device? If you are lean to start out with your device will add power that was lost due to it's poor initial state of tune otherwise.