I second using a little less fuel, even though it means you may have to add some more fuel half-way into the cooking if the temps start to drop.
Yeah you have to do that with chickens. Hard to make it all the way unless you stretch the time past the 1:20 it should normally take. Sometimes I start out with a chimney of charcoal and add lump to it half way through. Briquettes are cooler and start me off at a more ideal temp. The lump is a good way to keep things rolling given its quick ignition and higher heat. I add them at about :45. Of course, always add some smoking wood at both the start and again when you replensih if needed.