With tires of the same diameter as P225/75R-15s, a 3.23 will make about 25mph/1000rpm on the highway, with the normal 3-speed TF. That puts the engine at about 2800rpm at 70mph, which is on the bump of the torque curve for most B/RB engines, with stock-type cams and such.
You want highway cruising and "stop light" capabilities? Then a 3.73 with a OD automatic will do that. A thread of two of using later-model Chrysler 5+ speed automatics, most which have a 4.50 low gear and OD on the top side. Cost of adapters and the multiple pressure switches to make things work WILL be expensive, not to mention the cost of the upgraded rear axle gears. If you open up the rear axle, then you'll want a new Sure-Grip unit, new bearings, new seals, new wheel bearings, and all of that TOO, for good measure. PLUS getting the new driveshaft made and mounts for the newer trans procured. Maybe $4K+ by the time it's all done, or more?
A 3.73 by itself will certainly "wake things up", AND you'll hear it all of the time on the highway, too. Along with the much-decreased fuel economy ALL of the time!! What you now call "highway cruising" will become louder as the engine will be on the downside of the torque curve and headed toward the horsepower peak rpm. No "loafing" in that situation.
Tune the car to work as good as it can . . . now and with the later AVS2. Just remember that the secondaries in the carb will only open above about 2500rpm AND only at WOT, for best results. Adding a 4bbl alone will probably make little difference in low speed operation and power.
With a somewhat heavier car and highway gearing, it's usually best to not use WOT at lower road speeds, unless a WOT kickdown situation happens. Otherwise, use a bit less throttle input and keep ALL of the vacuum advance working as the centrifugal mechanical advance builds with rpm. WOT at lower speed loses the vacuum advance, so less power and more fuel use, by observation, especially when moving from a stopped position. LEARN how the equipment works best and then do things as it likes them, NOT how you perceive what it should be with what you have to work with. In other words, "realistic expectations".
Just some thoughts,
CBODY67
