The XE274 and vortecs heads will be fine for a daily driver in a 400. The truck in my avatar was my daily driver with a 406, Crane 284 Energizer cam, S/R Torquer heads, 670 Avenger, Performer RPM intake and a 2200 stall. Gas mileage was around 10 with 3.08's and 31" tires. It had a little more lope than the 268 cam that in the 350 when I first got the truck. Now that the truck has been more modified due to the need for horsepower along with 35's, it does not get driven that much on the street due to having to run a mix of pump gas and racing fuel. If you plan on hopping up in the future like most of us do, you might look at a set of 200cc heads. The 400's will easily take this size of runner without losing much torque.
As for the gears, I had both 3.42's with a t350 in a stock 81 Camaro Z with a 255/60 rear and 3.73 with a t-10 in a mildly-modified 78 Camaro Z with a 255/60 rear. Even though it has been ages ago, both were daily drivers while in Colorado making trips to the homeplace in Kansas. Gas mileage was very similar in both taking right at 2 tanks to get from Colorado Springs to Seneca (8 hr trip). Even though you are not planning on making many freeway miles, I would still go with the 3.42's. If you have a 350, I'd go with the 3.73's. The added torque of the 400 will make up the difference in the gearsets while keeping the revs lower just in case you go on the highway to help the 400 live longer.