Joe..couple things hre you want to keep in mind...the overall width flange to flange is the mounting surface on the axle faces...the measurement in backing plate to backing plate and the distance between the spring perch centers..
Know ahead what ratio you are looking for and know how to identify the axle in question in this manner...late models have tags on the axle cover bolt and also is on the engine compartment tag of trucks and vans..else it is on the build sheet but decoding is a must.
The track of your car is 56 inches..this is also important. (center of tread one tire to center of tread opposite tire) My Suburban has a 8 1/4 Dakota, I have a 9 1/4 under the Clup Coupe and 8 3/4 under the 41 Bz Cp The Suburban is the only one that will be close to stock and the Dakota axle and wheels fit like a glove...I got the 2:94 ratio and will be using the 4 speed auto here..no biggy as I also have the V6 pushing it. You may opt for the 3:23 or 3:55..think a lot of the 4WD had 3:79 ratios...the early Dakota will also yield matching bolt pattern. The 5th Ave and Diplomats use very high ratio rear gears, double isolated springs. Spongy setup..use your original springs and spring mounts and weld to newly selected axle..when measureing for placement of the mounts...meausure correctly..some pumpkins are offset...unequal axle lengths. centering to the backing plates will work every time...
Changing axles also brings up other probs...in using the Dakota I got smaller brakes..not what I wanted so I changed the backing plates from the 5th Ave..who's rear gear is 7 1/4 2:32 ratio but with the larger 2 1/4 10 inch brakes, they will bolt straight to the 8 1/4 Dakota gear.....take the entire backing plate and brake assy from one to the other.