2004 M75 with civilian steel driveshaft
2003 P71 police aluminum driveshaft

I already know about reusing the civilian yoke and ADTR spacer.

My question is: There are several different u-joints listed on RockAuto with different dimensions. Which two do I get to complete this swap?

Here is one Moog listing for example:
MOOG 331 Info
Fits
At Rear Axle; 1.188 IN Bearing Caps; Snap Ring Thickness .059"; with Steel Driveshaft; with 1-3/16 IN Bearing O.D.; with Aluminum Driveshaft; with 1.188 IN O.D. Bearing(s)
or
At Transmission; 1.188 IN Bearing Caps; Snap Ring Thickness .059"; with Steel Driveshaft; with 1-3/16 IN Bearing O.D.; with Aluminum Driveshaft; with 1.188 IN O.D. Bearing(s)

I've not done a u-joint before, but I figure I can watch enough how-to videos to do it and since the P71 shaft is already on the bench learning on how to do one end should be easy. I'm just not familiar with the measurements to select the right set of u-joints.

This will put me one step closer to completing my hybrid Grand Marquis-Interceptor project.

Thanks.

I would grab a pair of dial calipers and do some measuring to figure out what you're going to need to match up to the P71 driveshaft.Otherwise you're poking in the dark.

I use a ball joint press for servicing u-joints or if you have a huge heavy duty vise and the appropriate sized sockets to press and receive.You can also rent a ball joint press from a parts store.There are u-joint specific presses also that work really well but are expensive.Watch out for the hacks on youtube.

I chased a driveline vibration in my 2008 CVPI and ended up having a custom shaft built from steel. Absolutely ZERO performance difference between the two (other than getting rid of the vibration). Its a useless swap in my opinion.

Okay, I was hoping the u-joint was more standard; as in, only one size was used.

I'll just spring for a good u-joint puller and bearing cup installer and a set of pliers for the u-joint snap rings and take a measurement with my digital calipers.

I was hoping to not have to take the shaft off the car just to measure the joint at the yoke and then put it all back together again to keep driving the car then to later take it all off again to do the swap.

I was given the '03 P71 shaft from a friend who parted out another car, so it did not cost me anything. So I'm hoping the u-joint sizing matches up with the '04 civilian. I haven't seen any '04 P71s in the salvage yards for a long time.

Unfortunately there are about four different sizes of U-joints that were used...and the aluminum is different than the steel.
You'll most likely need a hybrid U-joint (has two different sizes on it).
But you should measure the existing U-joints to ensure you buy the proper one.

Thanks all. I just learned about the hybrid u-joint last night; found a YouTube vid where someone listed the exact part number NAPA SKF-UJ448 hybrid u-joint for this exact swap.

I found NAPA's site last night to be more helpful specifying aluminum vs steel and both sizes.

Just to make sure, I'll take a caliper measurement of the bearing caps.