slant daily driver buildup
Verdana, Can you say "Oxymoron"? Good mileage & power are mutually exclusive terms. Most likely you want a 'solid' slant 6 with a 2bbl carb to wring out the most power for the thrifty size of the engine. Trial & error, are the only sure ways to see if the power achieved by adding 2bbl carb to your 225 slant 6 engine is what you'll feel satisfied with. I'm guessing that a Mopar driver wants ultimate power...and will give up the economy to get it.
I respectively submit that good mileage and power are not mutually exclusive terms. The objective, efficiency, for most of us is to get the most power out of the fuel vapor we are able to create with our engine's induction system. It really doesn't matter if we have a 1 bbl, 4 bbl, or fuel injection for that induction system.
I got more power and better fuel mileage when I went to a full 2ΒΌ" exhaust system on my 73 /6. I didn't notice much difference around town, but added 2-3 mpg on the highway. Around town, I get 16 mpg. Mileage went from 22 to 25 with 28 being the best I ever got on the highway.
The car is a 73 Dart Custom that has a 225/904 and a 2.76:1 rear. The engine has 82K miles on it. The terrain for my highway trips is usually gently rolling. I usually drive an indicated 5 mph over the posted limit (to compensate for the error in the optimistic reading speedometer). My power test is top speed observed (as noted by the same inaccurate speedometer). TSO went from a little more than 95 to a very solid 105 mph. The original 1 bbl carburetor has since gone off, so more precise empirical data is not available.
A future plan for my ride is to have the cylinder head intake and exhaust passages cleaned up and the surface milled .050". These modifications should increase efficiency of the engine, adding power without decreasing fuel economy. All of this presumes, of course, that I don't change the way I drive this car.
I agree that more power is available for less money by going to a V-8 as I did with my originally /6 Demon. Starting with a $50 318 core, I built a 290 hp engine for less than $600. I would have to spend a LOT more to get that kind of power out of a /6.
Hope I've answered most of your questions and that you find my experience useful in your decision making process. Bon chance!