MPG Motor Magnum vs. Hemi

I think keeping the 413 and tuning it up really well (carb, ignition) should get highway mileage up into the high teens. Also going to larger wheels and tires will bump up your gearing a bit, 3.21 is just a hair on the short side for high-speed road tripping IMO. It's a tough call with the transmission though, personally I'm not a huge fan of the 518 I've heard they suck a ton of power and don't give the MPG improvements they should. Maybe a GM O/D trans or a Gear Vendors...?