Perhaps I'm commenting on the obvious. If so please forgive me but driveshafts don't spin at a constant velocity unless they have CV joints at each end. With a cross-type u-joint the variation in angular velocity within a given revolution increases as the driveshaft angle increases. Because of this even with all angles cancelling to zero, a steep driveshaft angle will create a noticeable vibration. So, careful placement of the rear crossmember is paramount irrespective of the carb flange. You may have to set the engine off center to match the rear axle to get horizontal angles correct.
As Spicer's tech tip says, U-joint angles need to be >1 degree to spread grease but they must be equal on both ends within 1 degree for trucks, 1/2 degree for motorhomes or they will vibrate, according to the book. I suppose motorhome drivers have less tolerance for vibration. To get the angles correct on my truck with an auxiliary trans I had to shim the front motor mount with front-end shims and with various washers on either end of the auxiliary. Now when the truck is loaded, the driveshaft is 1 degree off straight, main box through the auxiliary and same between the auxiliary and the rear axle. It does not vibrate even at 3,000 + RPM. You may have to change the shims between the rear axle and springs rather than shim the front motor mount. I wanted to avoid shimming the axle because I wanted the auxiliary springs to hit the perches correctly. You might also want to shim the carrier bearing. I have no idea if my carb flanges are level nor do I care. It's all about the u-joint angles.
