The first thing the OP needs to do is to find out whether the rear axle is centered in the chassis, and that won't happen without a measuring tape, and crawling around under the car.
You don't want to be adjusting the rear axle to fit the sheetmetal, you adjust sheetmetal to fit the chassis.
If the body isn't seated square, and your body mounts are able to be backed out, you could loosen the bolts, and use a come-along to pull the body to where it needs to be. However the rear axle placement needs to be confirmed beforehand.
