Your car is narrow, so you will need to find one from a mustang or camaro that is narrow enough to fit in there as stock or narrow a rear end to fit. Other considerations are the amount of power being applied and weight and traction of the car. If you don't make over 350 hp or so, many of the smaller rearends will work with a light car that does not run slicks.
Narrowing is not so big a deal. I started with a GM 12 bolt from a junk yard for $60 and narrowed it 8" at home. Ordered axles for $225 and bought a new posi unit from Powertrax for $275. This will withstand huge horsepower. Or you may get lucky with a first generation Camaro or Mustang that will slide right in there for $60.