The vortec heads along with the XE274H on a 400 should be a great combo. The 3.73 gears will be a good choice if you are going to be driving primarily in the city. If you are going to be doing any highway driving, I would recommend 3.55's. This motor would make so much torque, that there would probably not be a huge difference between the two gear ratios as far as overall 1/4 mile times go. Here are a couple of articles to look at and compare yours to. Keep in mind that you will have to get the modified Vortec heads to run the XE274H cam. Scoggin Dickey offers nice Modified Vortec combos.
Modified Vortec heads:
383, but it still gives you an idea what a 406 would do