I believe a slight declination toward the opposite end from the firebox helps distribute the heat evenly along the bottom of the reverse flow plate, this will also allow the drain to be at that end as well, when you do not want to use water, the drain can be open during the cooking cycle and excess grease/fat can drain off.
Your only talking 1" slope per 4' of plate, so effect on airflow should be mininum, but those are the two fundimentals of the reverse flow design that to me, make it an appealing design.
A good place to see an example and pics of a well designed reverse flow is to look at the Meadow Creek smokers sight, brose through the gallery of thier smoker's and you will see an awsome design for the reverse flow drain pan.
Good luck, and post some pics as you build.