It seems a few of you are interested in the details of this swap so I figured Id write up a bit of a walkthrough. I have no record of any measurements or anything, so this is more of a proof of concept. It can, in fact, be done for less than $400. Bear in mind that none of these pictures were taken with replicating the process in mind. Mostly just to post on Facebook and whine about how much of a pain in the ass it was during the whole 6 month adventure. Some background information; this 80 Volare is my daily. Slant 6 auto with 70k miles and dead reliable. Never planned to drag my deathtrap of a project S10 (LS swap in the works, 2.8 with a brake leak, super loose front end, and no power steering) out of the garage, but I had to. I had only anticipated driving it for about a month while I slapped the Plymouth back together, but other projects stretched that into 6 months. One day the 7 1/4 started dragging and banging around turns. Seeing how I NEVER checked and/or changed the fluid in it, I wasnt all too surprised. Figured Id nab an 8 1/4 and upgrade while I was at it. After some research, I settled on an 8 1/4 out of a Liberty with disk brakes and 3.73s. If I did it again, Id hunt down one with limited slip, but this rear was minty with zero rust and brand new brakes. It might have been easier to find a bolt up replacement out of another F-body, but this deep in the rust belt these cars are nowhere to be seen. First step was to remove all of the old mounting hardware. I mean ALL of it. nothing swaps over and theres a lot of junk welded/bolted the axle. Torches, hammers, and lots of grinding. The end result was a rear axle that was near identical to a 8 1/4 from the 70s. (Minus the disks) Before: After: The next step was to weld on the new 3" axle spring perches. Measure the width center-to-center of your old perches and use that measurement on the new one. Keep in mind that the 8 1/4 pumpkin is offset to one side unlike the 7 1/4. I measured from the backing plate on both sides to make sure they were even. After that, I went to work grinding down the calipers to fit under my 14" wheels. To clarify, 15" wheels SHOULD clear them. I just like my wheels. Plus I just put new tires on them and am broke af. I also rattlecanned it to seal it all up and make it look decent. Now to clean up all the old mounting hardware. I just wire wheeled most of the rust off mine to save cash, but I plan on buying an isoclamp delete when I put new 5 leaf springs and a sway bar under it. I initially planned to do all of it to begin with, but money held me back. All I could afford was new rubber and shocks. Bolt everything up! I used a motorcycle jack to roll it under. Highly recommend it. It should all bolt up exactly like the factory if your measurements are correct. Currently in the process of custom bending some rear brake line and adding a prop valve just in case. The ghetto clamped stock lines function perfectly, just look like crap. SIDE NOTE: DO NOT CUT BRAKE LINE FROM AXLE TO BODY ON NEW AXLE TO GET IT FROM A JUNKER! Mopar charges like $300 for that short line and theres no aftermarket solution. I work at a Ford/Mopar dealer and I couldnt get one for less. I cut mine without realizing it was a problem and ended up having to go back to the yard and grab one. DO NOT CUT THIS LINE! Last big hurdle was the driveshaft. Grab the one from the Liberty since youll need it. Mine came with the newer bolt on yoke, but not all do. Also, the newer Jeep diff has 29 splines over the 27 of the older diffs. The Jeep driveshaft and the F-body driveshaft have different size U-joints, so keep that in mind if you custom order a driveshaft to lenth. Im a cheap-ass, so my solution was to cut the old Plymouth shaft to length (1.6" off if I remember correctly), weld the Jeep end to it, and hope that it didnt vibrate to pieces. So far its been very smooth, but well see. A new one is always an option. Last step is to fix the speedo and its done! Im at this stage right now. Finding a gear with the teeth I needed in the F-body style was impossible so I swapped to the older metal housing that you can get gears for. Currently waiting on the cable and itll be buttoned up and back on the road. Any questions?