That metal there isnt thick enough to hold any power as you have found out. You can do like faster says and weld thick large washers to each side, or make a rear set up like I did, or, do exactly like Rick did and weld in metal plugs in the stampings. Any of the mods will work. You should try and get the rear mounting bolts where they were originally, unless your lower bars are adjustable your wheels could be not centered in the wheel well and make the rear axle not straight in the car. Measure each side from the center of the front spindle to the center of rear axle to get it even.
I would also weld the upper brackets to the floor around the outside as well, they are only spot welded from the factory and will be possibly the next failure to happen.
1972 hatchback, 28,000 orig miles, 427BBC/twin T4 turbo's/T56 six speed/big wheels, lowered down pro touring style-work in progress....
1973 vega wagon-under the blue flame knife.