When you are using a coilover on the rear the full weight of the car is on the shock mount. With the normal suspension the spring is in a different location, so the weight of the car does not need to be supported by the shock mount. Whether it fails has not much to do with how much power the car makes. It is about the weight and the impact the coilover, and thus the mount, will experience. If you're driving on a flat road it will be fine, but driving fast over big bumps can give a big impact.

On the rear you can make the Cosworth coilovers fit, just drill the hole in the arm from 10 to 12 mm and fit some washers to fill the gap. But it would probably be wise to strengthen the bottom mount on the suspension arm first.

On the front the 2wd Cosworth coilovers might work OK, if the height adjustment is long enough to compensate for the shorter struts. You could always fit Cosworth knuckles to the front which would solve the issue and give better handling as well.