Here's just a few steps along the way of trying to fit the lighter weight C4 batwing onto my car, which presently has the late C3 batwing attached to the older C3 iron differential.
The first thing needed was to make a 3/4" thick aluminum spacer to clear the C3 ring gear. The C4 cover is quite shallow compared to the C3 cover. Once there's clearance, I centered the batwing and put a couple tack welds on the spacer and cover to hold things together while I drilled the cover for the C3 pattern. Here it is bolted to an old spare 3.70 rear end I've used quite often for building and mocking things up. (The two parallel black marks near the outer mounts are explained later.)
Here's the separate pieces after welding up the original C4 bolt pattern holes in the cover.
And here's the cover a bit further down the process. Years ago I narrowed the C4 suspension on the car 3" to match the stock C3 trackwidth (so I can use stock C3 wheels on the street), so I had to cut (at the black marks) and reweld the the batwing after removing 1 1/2" on each end for rear knuckle clearance (the same as I had to do with the previous C3 batwing.
Here's the mostly finished cover bolted onto the mockup rear again. I welded the spacer to the cover both for aesthetics and so that I did not have two layers of cover gaskets that needed to seal. Due to clearance issues, and the desire to not have to do additional cutting and welding), I had to use a special small-head bolt in the upper right position to clear the plastic vent tube near that location. Also, I replaced the rubber bushings with new polyurethane units while it was handy.
Here's the initial test fit of the C4 batwing onto the differential that's in the car. Oh, I also had to weld/drill/tap a place in the cover for the lube plug, as the plug on a C4 is in the housing, not the cover like a C3. Thankfully everything fit. (I've got some steel material temporarily welded into a jig to keep the differential assembly from moving around until I can get the outer frame tab mounts fabricated and welded in.
Right now I'm still cutting and grinding to remove the remnants of the old frame tab mounts so I can weld in the new mounts.
This C4 batwing swapout will take an additional 5# off the car compared to the C3 batwing, which was a 15# reduction in weight compared to the original iron cover and steel crossmember.
Years ago, because I have a C4 suspension and a C3 differential and cover, I had to make several (steel) adapter brackets to mount the C4 spring and the C4 (and later, C6) toe rods. With the usage of the C4 cover, I should be able to delete most, if not all of those adapter brackets, resulting in an additional 6 pounds removed, for a net reduction of 11# with this conversion.