A question used to come up pretty often asking whether or not you can swap out steel driveshaft that comes on civilian models for a lighter aluminum shaft which is found in police models. The general consensus is yes you can, provided you replace the extension housing on the back of your transmission with a longer police version as well as swap the front yoke for a civilian yoke. Today I decided to take some measurements for curiosity sake and here are my findings.
First I should point out that the output shafts on the transmissions are different lengths. Civilian models have shorter output shafts and are actually preferred for high torque applications.
Now the driveshafts. Overall length of the driveshaft with yokes installed is basically the same between police and civilian models. Police shaft is a hair longer but it's negligible. With yoke removed police shaft is shorter.
As far as the yokes go... Civilian yoke has splines running from end to end. On police yoke the splines don't start until about half way down the tube.
When installing police shaft on a civilian model you will have approximately 2.375 inches of contact between driveshaft yoke and output shaft of the transmission. With steel civilian shaft you have 4.125 inches.
If you swap civilian yoke over to police shaft you will gain approximately another inch or so of contact area. This still doesn't give you as much contact area as stock civilian shaft. To achieve maximum contact install a pinion spacer.
This diagram compares different driveshafts and yokes and shows differences of contact areas.