So here's how the plan goes. My goal is total bolt in, no welding on the car, I wanted to see if I could do it without and still make it strong. I'm daily driving this and don't plan any 6000 rpm launches on drag slicks so I'm not going over the top.
I had a girdle water cut out of 1/4" steel that goes around the entire cover of the diff. The stock aluminum cover is replaced by steel one, I used chrome just because it cost $16 for a new one, cheaper than a wrecker (plus it looks nice). The cover is ground down in places and sandwiched between the girdle and the diff case. I may add some minor gussets where the girdle meets the square tube but it's a large cross section of plate and I don't think it will bend. 2"x2" square tube for arms with 2 1/2" x 1/4" wall mechanical tubing for the bushing mounts. The bushing are 2 piece, the lower half (nylotron) slips though the tube coming flush with the top. Then between the mount and the body a disk 5/8" thick, I made one set out of UHMW and one set out of hard rubber, I haven't decided which set to use yet but I'm leaning towards the rubber.
At the front I was having a hard time with the mounting options, only two options seemed to present themselves. One extend the diff off the front from the mounting ears to the stock Mazda pinion mount and put a bushing of some sort there. This option puts a lot of stress on the ears and creates a long lever which I didn't like. Option two was create some brackets that hard mount to the stock point and tie into the link bar at the rear of the crossmember. This looked better but Andrew pointed out it puts a big twisting force on the crossmember. Both options would only pick up one side of the diff at the front and trying to tie both sides together would make a dogs breakfast and look like crap. So I thought out of the box. The experience I gained prepping cars for stunts and special effects came to light. Car bodies are really strong where there are more than one layer and where ribs and bends are close together. One such place is at the rear of the tunnel where the rear seat seatbelts would mount, if you had them. Part two of the plan is to sandwich the body with a 2" x 3/8" bent bar below that picks up the two belt mounting holes and bolts through to a large 1/4 plate that goes over the top of the tunnel and along the top towards the rear to a third point that I drill about 4" back almost directly over the diff ears. Here I'll come down to the ears and add bushings etc. From shit I've done with cars in the past I feel this will be rock solid and will hold up over the long term no problem.
Rear mount
Front mount
revised: added stiffeners and capped the hollow tube.
Installed
The issues so far have been minor. After heat treating the stub shafts the bearing races were not up to factory hardness so I machined a shoulder and pressed a new needle bearing race on. JC Krueger copied my mounting system and had some trouble with the front mount bending so I added some stiffeners to it. He also claimed to get some movement in the tunnel under hard launches. Since I have a mild street car I'm not worried about this, I haven't noticed anything on my car after 5000 miles. I had a bit of trouble with venting but it seems to be fixed with a new baffle I made.
