Key question . . . which cam? What rpm for rated-peak torque?
With a 2.76 axle ratio, the effective ratio in 2nd is 4.000. Plug that gear ratio into the calculators and see what it comes to for 70mph. See how that plays into the mph/1000rpm calculator.
With a 2.76 axle ratio, it's comparatively got the 'OD" built-in, rather than having an extra OD gear and a 3.91 in 3rd and 2.76 effective ratio in OD.
Even IF you put an OD automatic in, you'd need about a 3.73 axle ratio in order to be able to use the OD on the highway, at normal speeds. End result, you're pretty much where you'd be with the deeper gear and OD, now. At NO additional cost.
With the H78-15/3.23 combination on my '70 383 "N" Monaco, I noticed that until it hit 62mph, the throttle response (using that famous "rch" measure), the response was "soggy", but got really "tight" at 62mph. Which would equate to about 2400rpm on the highway. The same rpm in the '66 Newport H78-14/2.76 is 68mph. Possibly that was the rpm where the converter dynamics had a greater degree of efficiency? On the '66, everything was the same at any highway speed.
Understand, too, that the '66 had the factory 2bbl, and the '70 had the factory AVS. The '66 had the "large" torque converter and the '70 had the "small" torque converter, so probably not an exact comparison per se, but is an indicator of the rpm levels on each car on the highway.
Given the dynamics of a normal street car, especially a heavier one with a stock stroke motor, that "passing gear" performance is more important to me than stop light grand prix performance. Certainly, off-idle response and power ARE important, but my priorities are more oriented toward "2 lane blacktop road" passing performance. Which allows 2nd gear to run out to about 90mph or more, quickly. With your 4bbl set-up, it would get there quicker than our '66 Newport 383 2bbl did.
Enjoy!
CBODY67