4 bbl intake 273

I see that you may be confused about gearing. The transmission has three gears which you cannot easily change, and you already know about them. They are 2.45,1.45,and 1.00. What these ratios do is multiply the engine's power by those numbers and send it on down to the rearend. Inside the rearend the power is multiplied some more,and then gets turned sideways, and finally sent out to the wheels.
Now, the rearend ratios are much more easily changed and come in about 5% to 10% steps, from 2.73 to well into the 5s This number means that whatever power is coming out of the transmission gets multiplied by this number, and is therfore called a ratio. So if 1 hp comes out of the transmission, in the rearend it can be multiplied by 2.73, or more, all the way to 5 times and more.
The penalty for all this multiplication,is that the rpm of the input to the rearend is also multiplied by this same ratio. So if the output rpm from the rear end is 1 rpm, then the input will be 2.73 rpm, or more,depending on what gear ratio is inside of it.
The rearend is always trading rpm input for power output.
So you need to decide how much rpm you are willing to put up with at 70 mph. This is your decision to make. From that decision, it possible to back-calculate what rearend ratio it takes to make that rpm. And from that ratio, it is possible to estimate a performance increase.
If the power curve of the engine is known, then it is possible to even calculate the power increase to the road, with the new rearend ratio.And the new road-power curve, will be a curve similar to the original, just more of it; higher up on the graph. Right from the stall speed to the blow-up rpm, the curves will be similar in shape. This you will really feel. Not like the 4 bbl which does not act this way.The 4bbl adds a little bit at the bottom rpm, when it begins to open, then adds more and more continuously as the rpm rises.
The only other thing that you can do to your engine that will make it feel this way, is to make it bigger, or to supercharge it.
Hence my allusion to the teener. And to cost.
Hope that helps