The 3 degree angle is just a good starting point. The 3 degrees is the right amount of angle for u-joints to work properly. Go to much more and they will bind up or wear to quickly, go less then and they could wear just as fast due to not rotating the bearings.
Set the motor in the frame and bolt it to the mounts. The frame should be setting at the approximate angle of a completed truck. You will have to guess at this, with a drop axle it will be lower in the front the normal. Now move the transmssion crossmember up or down to get the carburator level or slightly leaning back. This has nothing to with how it will run, its all about looks. Don't worry to much about this, just get it close, its the driveline angles that matter.
Once you have the engine where you want it, set the driveshaft in the rear axle yoke and transmission case. Measure the transmission angle then the driveshaft angle. They both should be pointing down. Subtract the driveline from the transmission and you better be in the 2 to 4 degree range. Now go to the rear axle and measure the driveline angle and the rear axle angle (use the yoke flange or pan gasket flange). These two should be pointing up the same amount as the trans is pointing down, with a 2 to 4 degrees u-joint angle.
Thats it, thats all that you need, the driveline angles, the motor and rear axle angles will be the same, one up the other down. It doesn't matter what they are as long as the u-joint angles are in the 2 to 4 degree range. In a ideal world, the motor would point down 3 degrees and the rear axle will point up three degrees with 2 degree u-joint angle.
Under a hard launch like a drag car, the axle will want to twist upwards so a little extra angle is added to the rear. This way under a load, the u-joint is still in its proper range. A easy going 6 cylinder parade truck will not have to worry about the extra torque angles, but if you plan to drive your truck hard add a extra bit to the rear axle angle.
To get the rear axle in the proper ride height, use ratchet tie strapes to pull the frame down to the axle to simulate the weight of the bed. Ask someone with a finished truck for the measurement. Your driveshaft angle will change a lot once you start adding weight. Joe
