Scott, it's best if the carburetor sits level on level ground, so that variations in attitude of the carb average out over the long run. The intake manifold has an angle built into it, which makes the engine sit down in the rear at that angle. Then the differential should be rotated to that angle or a little more, so the u-joints are working in the same plane under typical load.
Intake attitude doesn't matter on fuel-injected engines. On my LT1, there is no angle built into the intake. Still, the basic layout of front engine-rear drive vehicles tends to stay the same, so the engine needs to be down a bit in the rear to get the trans clear of the cab floor.
I wonder about the importance of matching u-joint angles, though. The common wisdom is that the joints won't wear properly and will cause vibration if they aren't running in close to the same plane, and that the bearings will brinnel if they don't have enough angle, but I've been looking at trucks with two-piece drivelines lately. There does not appear to be any effort to follow the "rules" in the trucks I've noticed. One medium duty truck (single rear axle) I checked out in a parking lot the other day looked to be as straight as a string from the trans through two drivelines to the rear end. I haven't heard that big rigs are prone to u-joint failure.
Any truck mechanics aboard who care to comment?
Joe
