I have a few questions to answer
1. set up shown is for urethane bushings or if you wish you could have the metal elastic like the originals
2. Yes it will fit a 289 chassis I started in 80 building cobras my original plan was to try to build something that was nice and close to the original yet economical that didn’t quite work out that was really nothing that you could use in the stock form so my first one or two cars with a 9 inch straight axle And mustang upper and lower control arms in the front then switched to tubular control arms full-size Ford spindles. did one chassis with the Corvette IRS with cobra suspension. then went to the 9 inch IRS don’t know why thought it was stronger which it is and never use the Corvette diff again
3. Never had a problem with
oil leaks in the C3 Diff. but everyone wanted the 9 inch.
4. Believe the C2 and C3 cast-iron cases use the same mounting the C3 aluminum cases have the front pinion mount cast. Ptrying to find a used alouminum case now to check that.
There are remanufactured C3 differentials with all new parts for $1800 and if you want to get more original Down the road. can take it out and put it in an aluminum Diff from cobra racing or Kirkam.
Remember I talk to my phone any miss spelled words or grammar it’s the phones fault
Tom