what type of car is it? do you intend to keep it purely strip, or are you going to drive it on the street as well? 403 is pretty big, if you don't need to over bore, i personally wouldn't overbore. (keep cylinder walls as thick as possible, but thats just me) i would look into forged parts, bigger cam, forced induction, proper tolerances, valve job, higher flow heads, etc. for gears, 4.11 or 4.56 if your only out for strip. they are not very good for mileage.
hope it made sense, post more info, and we will be able to help better. good luck on your project.